doc: add final full stop in CONTRIBUTING.md

PR-URL: https://github.com/nodejs/node/pull/3576
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

CONTRIBUTING.md

@@ -65,6 +65,7 @@ projects.  Do not send your patch to us, we cannot accept it.
 In case of doubt, open an issue in the
 [issue tracker](https://github.com/nodejs/node/issues/) or contact one of the
 [project Collaborators](https://github.com/nodejs/node/#current-project-team-members).
+
 Especially do so if you plan to work on something big. Nothing is more
 frustrating than seeing your hard work go to waste because your vision
 does not align with the project team. Node.js has two IRC channels,