grafana-pd-dashboard.md: 99% Completed commands duration description may be incorrect

Open grafana-pd-dashboard.md and read the two adjacent gRPC bullets in its gRPC section. Compare the English wording with the corresponding lines in docs-cn/grafana-pd-dashboard.md, then revise only the 99% Completed commands duration description to explain a duration or latency metric rather than a rate. Done means the English description matches the intended meaning and is distinct from the completed commands rate description.

What is changed, added or deleted? (Required)

In the gRPC section of grafana-pd-dashboard.md, two adjacent bullets have identical description text apart from the trailing "(P99)":

- Completed commands rate: The rate per command type at which gRPC commands are completed
- 99% Completed commands duration: The rate per command type at which gRPC commands are completed (P99)

The second panel's name is "99% Completed commands duration", but its description still talks about "rate", which may be incorrect for a duration/latency panel.

For reference, the Chinese source (docs-cn/grafana-pd-dashboard.md) describes these two panels differently:

- Completed commands rate：gRPC 命令的完成速率
- 99% Completed commands duration：99% 命令的最长消耗时间

The second line translates to roughly "99% Completed commands duration: the maximum time consumed by 99% of commands" — i.e., a duration/latency description, not a rate. Please double check whether the English description for "99% Completed commands duration" should instead describe a duration metric (e.g., similar in shape to "The time it takes for 99% of gRPC commands to complete"), using the Chinese doc as a reference for the intended meaning.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)

What is the related PR or file link(s)?

• Other reference link(s): found while reviewing the Japanese translation of this file (PR #23763); Chinese source at https://github.com/pingcap/docs-cn/blob/master/grafana-pd-dashboard.md

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch