doc: use sentence-casing for headers in collaborator guide

The collaborator guide uses both sentence case and headline case for
headings. Use sentence case to conform with our style guide.

PR-URL: https://github.com/nodejs/node/pull/34713


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>