doc: fix broken link in COLLABORATOR_GUIDE.md

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