Made a typo...

I was made an offer, and so I made a counter-offer. Instead of typing 'a', I typed 'an', at one point in the counter-offer.

The job requires 'excellent' writing skills, and I made sure to write 'excellently' in every other regard.

But I made the boo boo. I'm scared T______T

Is there anything I can do to rectify this? It would probably be incredibly unprofessional to send a follow up email with "a*", wouldn't it. I'm feeling sick in the stomach. Okay, a proper follow up that addresses the typo, saying how uncharacteristic it was of me to have made it – would this go amiss? Am I overthinking this?