[NOJIRA] Fix dead link to Github PR documentation

Closes #77.

Author: Giovds

README.md

@@ -85,7 +85,7 @@ Additional Resources
 + [Apache Maven Help JIRA project page][jira]
 + [Contributor License Agreement][cla]
 + [General GitHub documentation](https://help.github.com/)
-+ [GitHub pull request documentation](https://help.github.com/send-pull-requests/)
++ [GitHub pull request documentation](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork/)
 + [Apache Maven Twitter Account](https://twitter.com/ASFMavenProject)
 + #Maven IRC channel on freenode.org