This updates the documentation for `fs.watch()` to use `relativePath`
instead of `filename` in the recursive example, following the same
convention from PR #23990.
When `recursive: true` is set on `fs.watch()`, the callback receives a
relative path to the changed file rather than just a simple filename.
Using `relativePath` as the parameter name makes this distinction
clearer to users.
**Related to:** https://github.com/oven-sh/bun/pull/23990
Co-authored-by: Michael H <git@riskymh.dev>
