docs: fix Banking webhook HMAC example in README

validateBankingHMAC(hmacKey, hmacSign, ...) treats the first argument
as the signature and the second as the key, so the README example
passing (KEY, SIGNATURE) always returned false. Point the example at
the non-deprecated validateHMACSignature(hmacKey, hmacSignature, data)
and instantiate the validator, since the package exports the class.