Fix a deprecation message about a missing return type

[javiereguiluz]

This fixes the following deprecation:

  1x: Method "Symfony\Component\Config\Definition\ConfigurationInterface::getConfigTreeBuilder()" might add "TreeBuilder" as a native return type declaration in the future. Do the same in implementation "HtmlSanitizer\Bundle\DependencyInjection\Configuration" now to avoid errors or add an explicit @return annotation to suppress this message.
    1x in AddUserCommandTest::testCreateUserNonInteractive from App\Tests\Command

which we found when upgrading Symfony Demo app to the upcoming Symfony 5.4. See symfony/demo#1268 (comment)

[tgalopin]

Thanks!