fix(accordion): make header links keyboard accessible

Closes angular-ui#2869

Author: blah238

template/accordion/accordion-group.html

@@ -1,10 +1,10 @@
 <div class="panel panel-default">
   <div class="panel-heading">
     <h4 class="panel-title">
-      <a class="accordion-toggle" ng-click="toggleOpen()" accordion-transclude="heading"><span ng-class="{'text-muted': isDisabled}">{{heading}}</span></a>
+      <a href class="accordion-toggle" ng-click="toggleOpen()" accordion-transclude="heading"><span ng-class="{'text-muted': isDisabled}">{{heading}}</span></a>
     </h4>
   </div>
   <div class="panel-collapse" collapse="!isOpen">
 	  <div class="panel-body" ng-transclude></div>
   </div>
-</div>
+</div>