rm redundant role="button" from <button>

docs/_includes/js/dropdowns.html

@@ -122,7 +122,7 @@
   <p>Add <code>data-toggle="dropdown"</code> to a link or button to toggle a dropdown.</p>
 {% highlight html %}
 <div class="dropdown">
-  <button id="dLabel" type="button" data-toggle="dropdown" aria-haspopup="true" role="button" aria-expanded="false">
+  <button id="dLabel" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
     Dropdown trigger
     <span class="caret"></span>
   </button>