doc: add missing ) in CONTRIBUTING.md

PR-URL: #12444
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

CONTRIBUTING.md

@@ -62,7 +62,7 @@ does not align with the project team. (Node.js has two IRC channels:
 [#Node.js](http://webchat.freenode.net/?channels=node.js) for general help and
 questions, and
 [#Node-dev](http://webchat.freenode.net/?channels=node-dev) for development of
-Node.js core specifically.
+Node.js core specifically).
 
 For instructions on updating the version of V8 included in the *deps/*
 directory, please refer to [the Maintaining V8 in Node.js guide](https://github.com/nodejs/node/blob/master/doc/guides/maintaining-V8.md).