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: https://github.com/nodejs/node/pull/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>