doc: edit Landing Pull Requests

Edit the Landing Pull Requests section of the Collaborators Guide for
brevity and clarity.

PR-URL: https://github.com/nodejs/node/pull/26536


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>