doc: standardize on _pull request_

Sometimes we capitalize _pull request_ and sometimes we don't.
Standardize on lowercase based on Microsoft Style Guide, Chicago Manual
of Style, and GitHub's own docs and UI.

Refs: https://docs.github.com/en/github/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests
Refs: https://docs.github.com/en/github/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/merging-a-pull-request
Refs: https://docs.microsoft.com/en-us/style-guide/capitalization

PR-URL: https://github.com/nodejs/node/pull/39384


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>