Ooops...I goofed!

I would blame it on premarital confusion saying that you apologize for the mix up and gush over how you hope to see her at the wedding and look forward to meeting her in person. Then add on a little 'I promise to get your name right' comment at the end. I don't think it's that big a deal, gosh I hope your MIL isnt that picky...but I know some are! Don't feel bad, if that's the worst mistake you have made on them, I bet plenty of people can share other gaffs that they have made as well. Nobody's perfect!
My hairdresser was able to duplicate the style exactly based on the printout of the picture from this site.