util: update comment in util.promisify

child_process.exec has Symbol('util.promisify.custom') in its keys
and no longer has Symbol('customPromisifyArgs') in its keys, but it was
still used as an example of funtions with
Symbol('customPromisifyArgs').

PR-URL: https://github.com/nodejs/node/pull/25323


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>