doc: edit "Using git-node" section of Guide

Edit the "Using git-node" section of the Collaborator Guide. Make
sentences concise and clear, reduce use of passive voice, etc.

PR-URL: https://github.com/nodejs/node/pull/26580


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>