doc: typographical fixes in COLLABORATOR_GUIDE.md

Fix a typo and add a missing line break.

PR-URL: https://github.com/nodejs/node/pull/11163


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>