add example of callable action syntax

Author: taylorotwell

helpers.md

@@ -1014,6 +1014,8 @@ The `action` function generates a URL for the given controller action. You do no
 
     $url = action('HomeController@index');
 
+    $url = action([HomeController::class, 'index']);
+
 If the method accepts route parameters, you may pass them as the second argument to the method:
 
     $url = action('UserController@profile', ['id' => 1]);