docs(a): escape sample code in ng a directive

src/ng/directive/a.js

@@ -11,7 +11,7 @@
  *
  * The reasoning for this change is to allow easy creation of action links with `ngClick` directive
  * without changing the location or causing page reloads, e.g.:
- * <a href="" ng-click="model.$save()">Save</a>
+ * `<a href="" ng-click="model.$save()">Save</a>`
  */
 var htmlAnchorDirective = valueFn({
   restrict: 'E',