demo(alert): add btn-default class to button

src/alert/docs/demo.html

@@ -1,4 +1,4 @@
 <div ng-controller="AlertDemoCtrl">
   <alert ng-repeat="alert in alerts" type="alert.type" close="closeAlert($index)">{{alert.msg}}</alert>
-  <button class='btn' ng-click="addAlert()">Add Alert</button>
+  <button class='btn btn-default' ng-click="addAlert()">Add Alert</button>
 </div>