doc: s/origin/upstream/ collaborator guide

Use `upstream` to refer to `nodejs/node` instead of `origin`, because
that’s the more common setup.

PR-URL: https://github.com/nodejs/node/pull/12436


Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>