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

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

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.