doc: edit CONTRIBUTING.md for clarity

PR-URL: https://github.com/nodejs/node/pull/11045


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>