[ BUG ] Works on Linux but finding place holders fail on Darwin

Start with the gh-md-toc entry point and reproduce the three failing commands on Darwin using the supplied README example. Inspect the placeholder-detection and sed-related paths, then verify that all commands handle README.md correctly on Darwin without regressing Linux behavior.

./gh-md-toc --insert --no-backup --hide-footer --skip-header README.md
./gh-md-toc --no-backup --hide-footer --skip-header README.md
./gh-md-toc --hide-footer --skip-header README.md

The above commands all work on Linux but not Mac.

➜ cat README.md
My readme: https://github.com/sourcenetwork/defradb/blob/c1b612ea28b866093da2d7d1f473ea9de1072a93/README.md

➜ ./gh-md-toc --insert --no-backup --hide-footer --skip-header README.md

Table of Contents
=================

   * [Table of Contents](#table-of-contents)
   * [Install](#install)
   * [Key Management](#key-management)
   * [Start](#start)
   * [Configuration](#configuration)
   * [External port binding](#external-port-binding)
   * [Add a schema type](#add-a-schema-type)
   * [Create a document](#create-a-document)
   * [Query documents](#query-documents)
   * [Obtain document commits](#obtain-document-commits)
   * [DefraDB Query Language (DQL)](#defradb-query-language-dql)
   * [Peer-to-peer data synchronization](#peer-to-peer-data-synchronization)
   * [Securing the HTTP API with TLS](#securing-the-http-api-with-tls)
   * [Access Control System](#access-control-system)
   * [Supporting CORS](#supporting-cors)
   * [Backing up and restoring](#backing-up-and-restoring)
   * [Community](#community)
   * [Licensing](#licensing)
   * [Contributors](#contributors)
You don't have <!--ts--> or <!--te--> in your file...exiting
make[1]: *** [toc] Error 1
make: *** [docs] Error 2

Moreover, even without the --insert the command fails: ./gh-md-toc --no-backup --hide-footer --skip-header README.md

Table of Contents
=================

   * [Table of Contents](#table-of-contents)
   * [Install](#install)
   * [Test Title](#test-title)
   * [Key Management](#key-management)
   * [Start](#start)
   * [Configuration](#configuration)
   * [External port binding](#external-port-binding)
   * [Add a schema type](#add-a-schema-type)
   * [Create a document](#create-a-document)
   * [Query documents](#query-documents)
   * [Obtain document commits](#obtain-document-commits)
   * [DefraDB Query Language (DQL)](#defradb-query-language-dql)
   * [Peer-to-peer data synchronization](#peer-to-peer-data-synchronization)
   * [Securing the HTTP API with TLS](#securing-the-http-api-with-tls)
   * [Access Control System](#access-control-system)
   * [Supporting CORS](#supporting-cors)
   * [Backing up and restoring](#backing-up-and-restoring)
   * [Community](#community)
   * [Licensing](#licensing)
   * [Contributors](#contributors)
You don't have <!--ts--> or <!--te--> in your file...exiting
make[1]: *** [toc] Error 1
make: *** [docs] Error 2

And lastly also without both --insert and --no-backup flags: ./gh-md-toc --hide-footer --skip-header README.md:

Table of Contents
=================

   * [Table of Contents](#table-of-contents)
   * [Install](#install)
   * [Test Title](#test-title)
   * [Key Management](#key-management)
   * [Start](#start)
   * [Configuration](#configuration)
   * [External port binding](#external-port-binding)
   * [Add a schema type](#add-a-schema-type)
   * [Create a document](#create-a-document)
   * [Query documents](#query-documents)
   * [Obtain document commits](#obtain-document-commits)
   * [DefraDB Query Language (DQL)](#defradb-query-language-dql)
   * [Peer-to-peer data synchronization](#peer-to-peer-data-synchronization)
   * [Securing the HTTP API with TLS](#securing-the-http-api-with-tls)
   * [Access Control System](#access-control-system)
   * [Supporting CORS](#supporting-cors)
   * [Backing up and restoring](#backing-up-and-restoring)
   * [Community](#community)
   * [Licensing](#licensing)
   * [Contributors](#contributors)
You don't have <!--ts--> or <!--te--> in your file...exiting

➜ ./gh-md-toc --version

0.10.0

os:     Darwin
arch:   arm64
kernel: 23.4.0
shell:  GNU bash, version 5.2.26(1)-release (aarch64-apple-darwin23.2.0)
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

curl : curl 8.4.0 (x86_64-apple-darwin23.0) libcurl/8.4.0 (SecureTransport) LibreSSL/3.3.6 zlib/1.2.12 nghttp2/1.58.0
wget : not installed
grep : grep (BSD grep, GNU compatible) 2.6.0-FreeBSD
awk  : awk version 20200816
sed  : sed: illegal option -- -
usage: sed script [-Ealnru] [-i extension] [file ...]
        sed [-Ealnu] [-i extension] [-e script] ... [-f script_file] ... [file ...]