doc: fix `prohibited-strings` warning in `pull-requests.md`
corrected "Github" to "GitHub" in compliance to the linting standards
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Warn log
$ node tools/lint-md.js -q -f doc/guides/
doc/guides/contributing/pull-requests.md
87:1-88:19 warning Use "GitHub" instead of "Github" prohibited-strings remark-lint
⚠ 1 warning