doc: update parameter type for fs.chmod()

The mode parameter of fs.chmod() and fs.chmodSync() can be a string or
an integer. This change updates the documentation, which currently omits
string as a valid type for mode.

PR-URL: #31085
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/fs.md

@@ -1380,7 +1380,7 @@ changes:
 -->
 
 * `path` {string|Buffer|URL}
-* `mode` {integer}
+* `mode` {string|integer}
 * `callback` {Function}
   * `err` {Error}
 
@@ -1457,7 +1457,7 @@ changes:
 -->
 
 * `path` {string|Buffer|URL}
-* `mode` {integer}
+* `mode` {string|integer}
 
 For detailed information, see the documentation of the asynchronous version of
 this API: [`fs.chmod()`][].