fix(alerting): fix broken model bind in alerting tab

public/app/plugins/panel/graph/partials/tab_alerting.html

@@ -53,7 +53,7 @@
     <div class="gf-form">
       <span class="gf-form-label width-12">Frequency (seconds)</span>
       <input class="gf-form-input max-width-10" type="number"
-        ng-model="ctrl.panel.alerting.interval" placeholder="60"></input>
+        ng-model="ctrl.panel.alerting.frequency" placeholder="60"></input>
     </div>
   </div>
   <div class="gf-form-group section">