ExceptionHandlerExamplesSpec in docs cannot be copy / pasted

Some examples are broken again.

The example doesn't compile. The problem that was addressed in this PR needs to be addressed again in the same way. Essentially myExceptionHandler cannot be a top-level entity with respect to a file (in Scala 2), it should be in MyApp. The specification compiles because this snippet is itself in MyExplicitExceptionHandler, but a user looking at the documentation is unlikely to figure that out.

Originally posted by @FlorianCassayre in https://github.com/akka/akka-http/issues/1465#issuecomment-993396956