docs(guide/Developer Guide): Update twitter handle

Replaced the old Angular twitter handle with the new one.

Closes #15903

Author: vigneshTheDev

docs/content/guide/index.ngdoc

@@ -75,7 +75,7 @@ Official announcements, news and releases are posted to our blog, G+ and Twitter
 
 * [AngularJS Blog](http://blog.angularjs.org/)
 * [Google+](https://plus.google.com/u/0/+AngularJS)
-* [Twitter](https://twitter.com/angularjs)
+* [Twitter](https://twitter.com/angular)
 * [AngularJS on YouTube](http://youtube.com/angularjs)
 
 ## Contributing to AngularJS