doc: use "repository" in maintaining-V8 doc

Use "repository" instead of "repo". This also adjusts capitalization in
a header that was already being modified to use "repository" because
even I couldn't bring myself to put that in a separate commit.

PR-URL: https://github.com/nodejs/node/pull/39179


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>