doc: update parameter type for fsPromises.chmod()

This was overlooked in https://github.com/nodejs/node/pull/31085.

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>