fix(button-demo.html): icon change for rtl rendering

icon change from check circle to check to fix rtl display

Author: RichOren

src/demo-app/button/button-demo.html

@@ -3,21 +3,21 @@
     <button md-button>flat</button>
     <button md-raised-button>raised</button>
     <button md-fab>
-      <i class="material-icons md-24">check circle</i>
+      <i class="material-icons md-24">check</i>
     </button>
     <button md-mini-fab>
-      <i class="material-icons md-24">check circle</i>
+      <i class="material-icons md-24">check</i>
     </button>
   </section>
 
   <section>
     <a href="http://www.google.com" md-button color="primary">link</a>
     <a href="http://www.google.com" md-raised-button>link</a>
     <a href="http://www.google.com" md-fab>
-      <i class="material-icons md-24">check circle</i>
+      <i class="material-icons md-24">check</i>
     </a>
     <a href="http://www.google.com" md-mini-fab>
-      <i class="material-icons md-24">check circle</i>
+      <i class="material-icons md-24">check</i>
     </a>
   </section>
 
@@ -55,7 +55,7 @@
     <a href="http://www.google.com" md-button color="accent" [disabled]="isDisabled">off</a>
     <button md-raised-button color="primary" [disabled]="isDisabled">off</button>
     <button md-mini-fab [disabled]="isDisabled">
-      <i class="material-icons md-24">check circle</i>
+      <i class="material-icons md-24">check</i>
     </button>
   </section>
 </div>