doc: fix broken link in COLLABORATOR_GUIDE.md

PR-URL: nodejs/node#10337
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

COLLABORATOR_GUIDE.md

@@ -36,7 +36,7 @@ Collaborators or additional evidence that the issue has relevance, the
 issue may be closed. Remember that issues can always be re-opened if
 necessary.
 
-[**See "Who to CC in issues"**](./onboarding-extras.md#who-to-cc-in-issues)
+[**See "Who to CC in issues"**](./doc/onboarding-extras.md#who-to-cc-in-issues)
 
 ## Accepting Modifications