Let it ride! These things are very common, and even the "perfect" examples in the essays in "How to get into Top MBA Programs" had typos and minor errors in them. Adcoms are too busy to be acknowledging things that aren't directly related to your candidacy. If you forgot to add an entire job or something major, tell them. But I'm guessing that they won't really care if you write to apologize for spelling a word wrong.

I feared the same thing after I made some pretty major mistakes when I was rushing to meet UCLA's deadline by around 30 seconds. I discovered the next day that an essay that I had been adapting from other's contained the exact same sentence twice in the same essay AND my final optional essay read this: "I hope my choice of recommenders will be most helpful to you in determining my suitability and fit at Stanford GSB." I was so rushed that I never changed the school name for the pdf that I uploaded. Classic mistake! That said, I was still offered an interview. I hope this means that they just looked past it, but I'll find out for sure next week with a decision.

I wouldn't worry too much. Good luck!