fix annotation editor error

public/app/plugins/datasource/prometheus/partials/annotations.editor.html

@@ -2,7 +2,7 @@
 	<div class="section">
 		<h5>Search expression</h5>
 		<div class="editor-option">
-			<input type="text" class="span6" ng-model='annotation.expr' placeholder="ALERTS"></input>
+			<input type="text" class="span6" ng-model='ctrl.annotation.expr' placeholder="ALERTS"></input>
 		</div>
 	</div>
 </div>
@@ -12,17 +12,17 @@
 		<h5>Field formats</h5>
 		<div class="editor-option">
 			<label class="small">Title</label>
-			<input type="text" class="input-small" ng-model='annotation.titleFormat' placeholder="alertname"></input>
+			<input type="text" class="input-small" ng-model='ctrl.annotation.titleFormat' placeholder="alertname"></input>
 		</div>
 
 		<div class="editor-option">
 			<label class="small">Tags</label>
-			<input type="text" class="input-small" ng-model='annotation.tagKeys' placeholder="label1,label2"></input>
+			<input type="text" class="input-small" ng-model='ctrl.annotation.tagKeys' placeholder="label1,label2"></input>
 		</div>
 
 		<div class="editor-option">
 			<label class="small">Text</label>
-			<input type="text" class="input-small" ng-model='annotation.textFormat' placeholder="instance"></input>
+			<input type="text" class="input-small" ng-model='ctrl.annotation.textFormat' placeholder="instance"></input>
 		</div>
 	</div>
 </div>