Update FOSRestExtension.php

Fixed fos_rest.converter.request_body.validation_errors_argument argument if validate param of body_converter is false.

Avoid Fatal error: Uncaught exception 'Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException' with message 'The service "fos_rest.converter.request_body" has a dependency on a non-existent parameter "fos_rest.converter.request_body.validation_errors_argument".' in /home/valentin/web2/akinator_api/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php on line 108