typo in the comment

[Александр Кизеев <kizalvic [at] gmail.com>]

Hi. There's a typo in the comment: 

https://github.com/arthurdejong/python-stdnum/blob/master/stdnum/de/vat.py#L22. 

Further in the code everything is fine: 

https://github.com/arthurdejong/python-stdnum/blob/master/stdnum/de/vat.py#L55