doc: fix broken link in pull-requests.md

PR-URL: #18873
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: justin0022

doc/guides/contributing/pull-requests.md

@@ -660,4 +660,4 @@ you can take a look at the
 [guide for writing tests in Node.js]: ../writing-tests.md
 [https://ci.nodejs.org/]: https://ci.nodejs.org/
 [IRC in the #node-dev channel]: https://webchat.freenode.net?channels=node-dev&uio=d4
-[Onboarding guide]: ../onboarding.md
+[Onboarding guide]: ../../onboarding.md