doc: update mode type for mkdir() functions

This commit updates the documentation for fs.mkdir(),
fs.mkdirSync(), and fsPromises.mkdir() to reflect the fact
that their mode option can be a string.

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>