Fix mistranslation "Lieferantennauftrag" -> "Bestellung"

There is the strange term of "Lieferantenauftrag" that has been used for
"Purchase Order" which translates to "Supplier Order". This is not
standard at all and should be fixed to "Bestellung".

A Google search for "Bestellung" returns 97 million results, a search
for "Lieferantenauftrag" returns 972 results so it should be clear what
is standard.

Author: Dominik Ottenbreit