[Config] Add missing paranthesis of arrayPrototype method

Author: issei-m

components/config/definition.rst

@@ -555,7 +555,7 @@ tree with ``append()``::
             ->isRequired()
             ->requiresAtLeastOneElement()
             ->useAttributeAsKey('name')
-            ->arrayPrototype
+            ->arrayPrototype()
                 ->children()
                     ->scalarNode('value')->isRequired()->end()
                 ->end()