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

@@ -218,4 +218,4 @@ CoC](http://www.rust-lang.org/conduct.html).
   attention-stealing behavior is not welcome.
 * Avoid the use of personal pronouns in code comments or
   documentation. There is no need to address persons when explaining
-  code (e.g. "When the developer")
+  code (e.g. "When the developer").