Progressbar: Add link to custom label demo in demo index.

Author: kborchers

demos/progressbar/index.html

@@ -9,6 +9,7 @@
 <ul>
 	<li><a href="default.html">Default functionality</a></li>
 	<li><a href="indeterminate.html">Indeterminate</a></li>
+	<li><a href="label.html">Custom Labels</a></li>
 </ul>
 
 </body>