doc: simplify nomination process text

In GOVERNANCE.md, simplify the text describing Collaborator nominations.

PR-URL: https://github.com/nodejs/node/pull/27317


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>