Update the CONTRIBUTING.md file

CONTRIBUTING.md

@@ -163,7 +163,7 @@ $ invoke clean
 
 [2]: https://virtualenv.pypa.io
 [3]: http://www.pyinvoke.org
-[git]: http://code.ubuntu-it.org/ubuntu-it-web/www/wikis/git-setup
+[git]: http://code.ubuntu-it.org/ubuntu-it-web/www/blob/docs/docs/intro-to-git.md
 [ml]: http://liste.ubuntu-it.org/cgi-bin/mailman/listinfo/gruppo-web
 [irc]: http://chat.ubuntu-it.org/#ubuntu-it-web
 [vanilla]: https://github.com/ubuntudesign/ubuntu-vanilla-theme