Withdraw your application and apply next year. By writing a letter to the schools to somehow add to your application without going thru AMCAS would be a violation on the schools part. Additionally, it would just be pointing out that you are so sloppy, unprepared, lack attention to detail, that even the most cursory proofreading would have caught this. If you dont even care enough to be careful in applying, what does that say about your competency to be a careful physician? I would withdraw now before verification so you do not become a reapplicant at all the schools next cycle. You will lose all your money but you also will be wasting more with secondaries. Take the pain and disappointment now and move on. This is about as bonehead an error that is possible and no medical school is gonna admit a bonehead. Get it withdrawn before it becomes known at all the schools
And now I will rant here generally on this. Far, far too often I am utterly astounded that the best and the brightest, the premeds, cant even be careful in their application. I find many have not even asked anyway to proofread it before submitting. I find many do not print it off in a pdf and proofread themselves. I advise all students to: 1) have at least 2 proofreaders; 2) to print off via PDF and proofread it; and 3) read it out loud together with a proofreader. That last one is vital and I mean EVERY letter, word, date, title, hour. You will be amazed what you catch from missing a coma to incorrect hours to see just how bad some run-on sentence reads