doc: add missing URL types in fs promise API

This seems to be slipped in https://github.com/nodejs/node/pull/18297

PR-URL: https://github.com/nodejs/node/pull/18599
Refs: https://github.com/nodejs/node/pull/18309


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>