[Bug] Description of rewrite_file_index procedure is confusing

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Paimon version

Master

### Compute Engine

Flink

### Minimal reproduce step

Based on the results of local tests, the description here should be:`rewrite the file index for the specified partition in the table`

### What doesn't meet your expectations?

Confusing description

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start by locating the rewrite_file_index procedure and reviewing its current description alongside the issue's local-test result. Update the description to state that it rewrites the file index for the specified partition in the table, then verify the wording against the reported Flink reproduction.

Written by the indexing model from the issue text.