Update Github links on homepage.

The Github links on the homepage were pointing to the old "armstrongcms" Github account, which was retired 2 years ago (see https://groups.google.com/d/topic/armstrongcms/pbRPz2OlE_8/discussion). This error was introduced in commit 6892033e94bd2f46de1c01e156f4598608743fa0.