Update Google+ link to GitHubGuides

* Google+ link should be GitHubGuides, not just GitHub, so that training videos are the first thing seen.
* Camelcase all social media links for easier reading of "GitHub" with the right capitalization.

Author: Matthew McCullough

index.html

@@ -126,9 +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="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>
+    <a href="https://facebook.com/GitHub"><span class="octicon octicon-mark-facebook"></span></a>
+    <a href="https://plus.google.com/+GitHubGuides"><span class="octicon octicon-mark-google"></span></a>
+    <a href="https://twitter.com/GitHubTraining"><span class="octicon octicon-mark-twitter"></span></a>
   </div>
 </section>