doc: update Collaborator Guide reference

In pull-requests.md:

* Refer to the Collaborator Guide as Collaborator Guide and not
  Collaborator's Guide. That is how the doc describes itself and we
  should be consistent.

PR-URL: https://github.com/nodejs/node/pull/20473


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>