doc: edit "How Can I Help?" in Collaborator Guide

Edit the "How Can I Help?" section of the Collaborator Guide to keep
sentences simple, short, and direct.

PR-URL: https://github.com/nodejs/node/pull/26895


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>