fix docs about disabling regex in example.json

[langfingaz]

In https://github.com/bnw/firefly-iii-fints-importer/blob/0192cb8995c376e584dbeba3bc419fe662b1cb50/app/TransactionsToFireflySender.php#L95C1-L95C59, we check if both, regex_match and regex_replace are empty, not if they are null. Thus I propose to change the documentation inside example.json.

[bnw]

I think due to this null probably also worked?
Nevertheless, your suggestion is definitely an improvement. Thanks a lot :)