docs(services): use $log service in example consistently with $log API

The $log provider returns an object and not a function, so this example, which appears to be using
the $log provider, should call it as it would be called in a real-world application.

Closes angular#5875

Author: joshuaflanagan

docs/content/guide/dev_guide.services.managing_dependencies.ngdoc

@@ -55,7 +55,7 @@ of which depend on other services that are provided by the Angular framework:
 
       function log() {
         if (messageQueue.length) {
-          $log('batchLog messages: ', messageQueue);
+          $log.log('batchLog messages: ', messageQueue);
           messageQueue = [];
         }
       }