Just re-enter on the US passport. 50/50 chance they ask anything. If they do you can either try to explain that you made a mistake or you could just claim that you thought you left on the US passport but you showed your Korean one to the check in agent as well and they must have put the wrong info into the system opps-so-sorry-they-caused-this-problem.

Personally, I'd go for option two as it's more likely to work and less likely to cause US CBP to screw you around as they are wont to do when given the chance. "Trying to explain" is worst possible thing to do if you haven't been asked yet. YMMV, obviously.