docs(watch): use relativePath parameter name in recursive example (oven-sh#24716)

This updates the documentation for `fs.watch()` to use `relativePath`
instead of `filename` in the recursive example, following the same
convention from PR oven-sh#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:** oven-sh#23990

Co-authored-by: Michael H <git@riskymh.dev>

Author: braden-w

docs/guides/read-file/watch.mdx

@@ -23,8 +23,8 @@ To listen to changes in subdirectories, pass the `recursive: true` option to `fs
 ```ts
 import { watch } from "fs";
 
-const watcher = watch(import.meta.dir, { recursive: true }, (event, filename) => {
-  console.log(`Detected ${event} in ${filename}`);
+const watcher = watch(import.meta.dir, { recursive: true }, (event, relativePath) => {
+  console.log(`Detected ${event} in ${relativePath}`);
 });
 ```