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

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

PR-URL: #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>

doc/guides/contributing/pull-requests.md

@@ -84,7 +84,7 @@ You can use any name/email address you prefer here. We only use the
 metadata generated by `git` using this configuration for properly attributing
 your changes to you in the `AUTHORS` file and the changelog.
 
-If you would like for the Github UI to link the commit to your account
+If you would like for the GitHub UI to link the commit to your account
 and award you the `Contributor` label after the changes have been merged,
 make sure this local email is also added to your
 [GitHub email list](https://github.com/settings/emails).