Fixed singlestat mapping typo (#5070)

public/app/plugins/panel/singlestat/mappings.html

@@ -12,7 +12,7 @@
   </div>
 </div>
 <div class="editor-row" ng-if="ctrl.panel.mappingType==1">
-  <h5 class="page-heading">Set valuea mappings</h5>
+  <h5 class="page-heading">Set value mappings</h5>
   <div class="gf-form-group">
     <div class="gf-form" ng-repeat="map in ctrl.panel.valueMaps">
       <span class="gf-form-label">