doc: update mode type for fchmod() functions

fs.fchmod() and fs.fchmodSync() both support strings as their
mode argument. This commit updates the documentation to
reflect this.

PR-URL: #31115
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/fs.md

@@ -1979,7 +1979,7 @@ changes:
 -->
 
 * `fd` {integer}
-* `mode` {integer}
+* `mode` {string|integer}
 * `callback` {Function}
   * `err` {Error}
 
@@ -1992,7 +1992,7 @@ added: v0.4.7
 -->
 
 * `fd` {integer}
-* `mode` {integer}
+* `mode` {string|integer}
 
 Synchronous fchmod(2). Returns `undefined`.