Use the correct XML namespace

replaces #20
actually DoctrineBundle uses schema, not schemas: https://github.com/doctrine/DoctrineBundle/blob/master/DependencyInjection/DoctrineExtension.php#L742 and neither does https://github.com/doctrine/DoctrinePHPCRBundle/blob/master/DependencyInjection/DoctrinePHPCRExtension.php#L515