doc: fix `prohibited-strings` warning in `pull-requests.md`

Correct "Github" to "GitHub" in compliance
to the linting standards.

PR-URL: https://github.com/nodejs/node/pull/28127


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>