Update PageTitleExtension.php to remove deprecatd message in symfony 5.4

[Nakira]

Upgrading to symfony 5.4, this removes this depreciation message:

Method "Twig\Extension\ExtensionInterface::getFunctions()" might add "array" as a native return type declaration in the future. Do the same in implementation "Mobizel\Bundle\MarkdownDocsBundle\Twig\Extension\PageTitleExtension" now to avoid errors or add an explicit @return annotation to suppress this message.