docs(contribute): remove the word beta

closes #1975

Author: wajdibr

public/contribute.jade

@@ -5,7 +5,7 @@
   .l-sub-section
     h3 Angular 2
 
-    p Angular 2, now in beta, is a next generation mobile and desktop application development platform.
+    p Angular 2 is a next generation mobile and desktop application development platform.
 
     a(href="https://github.com/angular/angular/blob/master/CONTRIBUTING.md" class="button" md-button) Contribute to Angular 2