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>