No, it's not true. I've looked for it, too, and it doesn't appear anywhere she's supposed to have said it. Or in any other 18th century sources that google can find, for that matter. Usually she's claimed to have said it when asked to comment on her sentence at the end of the trial, which would make sense with the content of the quote, but she didn't actually say anything according to the protocols. There's nothing like that quote in any other part of the trial, either. If not at the trial it's attributed to her at the scaffold, but the things she did and said were closely reported on and there's nothing resembling that quote. It would also be less logical to say that outside of the context of her sentencing.