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