#74: fix BY iban structure

[ccostin93]

fixes https://github.com/arturmkrtchyan/iban4j/issues/74

see https://bank.codes/iban/structure/belarus/ The strange thing is that the best was already correct.

{new Iban.Builder()
    .countryCode(CountryCode.BY)
    .bankCode("NBRB")
    .branchCode("3600") // <-- branchCode was given, not accountType
    .accountNumber("900000002Z00AB00")
    .build(), "BY13NBRB3600900000002Z00AB00"},