Update twig filter declaration

[taavit]

see https://scrutinizer-ci.com/g/taavit/TwigExtraBundle/inspections/463d8165-f6c4-4129-a762-7728fa6c3d38/issues/files/Twig/Extension/DateExtension.php?status=existing&orderField=path&order=asc#inspectioncomment-32870973