Add rule BR-DE-26 to UBL CreditNote

The rule BR-DE-26 is currently only implemented for UBL Invoice and CII, but missing in UBL CreditNote. Since the document type code 384 is called "Corrected Invoice", but there is no type "Corrected CreditNote" in the list https://unece.org/fileadmin/DAM/trade/untdid/d16b/tred/tred1001.htm - I would recommend to add the rule "BR-DE-26" also to UBL Credit Note, to allow the possibility for "Corrected CreditNotes".