doc: fix a typo in the CONTRIBUTING.md

Delete a typo dot in the link to stability index.

PR-URL: https://github.com/nodejs/node/pull/6343


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>