doc: fix missing newline character

PR-URL: #16447
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

COLLABORATOR_GUIDE.md

@@ -671,4 +671,4 @@ LTS working group and the Release team.
 [backporting guide]: doc/guides/backporting-to-release-lines.md
 [Stability Index]: doc/api/documentation.md#stability-index
 [Enhancement Proposal]: https://github.com/nodejs/node-eps
-[git-username]: https://help.github.com/articles/setting-your-username-in-git/
+[git-username]: https://help.github.com/articles/setting-your-username-in-git/