doc: standardize on not capitalizing _collaborator_

Sometimes we capitalize _collaborator_ and sometimes not. After
consulting the Microsoft Style Guide and The Chicago Manual of Style,
I've concluded it is best to not capitalize it. For consistency, apply
that to our various .md files.

Refs: https://docs.microsoft.com/en-us/style-guide/capitalization

PR-URL: https://github.com/nodejs/node/pull/39379


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>