Graph Panel: Fixed typo in thresholds form (#21903)

public/app/plugins/panel/graph/thresholds_form.html

 <div class="gf-form-group">
   <p class="muted" ng-show="ctrl.disabled">
     Visual thresholds options <strong>disabled.</strong>
-    Visit the Alert tab update your thresholds. <br>
+    Visit the Alert tab to update your thresholds.<br>
     To re-enable thresholds, the alert rule must be deleted from this panel.
   </p>
   <div ng-class="{'thresholds-form-disabled': ctrl.disabled}">