Add Google+ icon link

Author: jordanmccullough

index.html

@@ -126,8 +126,9 @@ <h2>Let's be social</h2>
       Stay up to date with the latest GitHub Training events.
     </p>
 
-    <a href="https://facebook.com/github"><span class="mega-octicon octicon-mark-facebook"></span></a>
-    <a href="https://twitter.com/githubtraining"><span class="mega-octicon octicon-mark-twitter"></span></a>
+    <a href="https://facebook.com/github"><span class="octicon octicon-mark-facebook"></span></a>
+    <a href="https://plus.google.com/+GitHub"><span class="octicon octicon-mark-google"></span></a>
+    <a href="https://twitter.com/githubtraining"><span class="octicon octicon-mark-twitter"></span></a>
   </div>
 </section>