minor #8641 [Config] Add missing paranthesis of arrayPrototype method (issei-m)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #8641).

Discussion
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[Config] Add missing paranthesis of `arrayPrototype` method

Commits
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755a2bf [Config] Add missing paranthesis of `arrayPrototype` method

Author: xabbuh

components/config/definition.rst

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