docs(guide/Services): improve the code example

A factory that doesn't return anything is a bad example of a factory.

Closes #14139

Author: thorn0

docs/content/guide/services.ngdoc

@@ -138,9 +138,13 @@ batchModule.factory('batchLog', ['$interval', '$log', function($interval, $log)
  */
 batchModule.factory('routeTemplateMonitor', ['$route', 'batchLog', '$rootScope',
   function($route, batchLog, $rootScope) {
-    $rootScope.$on('$routeChangeSuccess', function() {
-      batchLog($route.current ? $route.current.template : null);
-    });
+    return {
+      startMonitoring: function() {
+        $rootScope.$on('$routeChangeSuccess', function() {
+          batchLog($route.current ? $route.current.template : null);
+        });
+      }
+    };
   }]);
 
 ```