Send every school you applied to an email with the subject line: correction to AMCAS application. Say, "as you know, no change can be made to the AMCAS application after it is submitted. It has recently come to my attention that a speeding ticket I received in [state] in [month/year] for 71 mph in a 60 mph zone is classified as a misdemeanor and should have been reported as such on the AMCAS application. I apologize for any inconvenience this may cause. Thank you for considering my application."
I would not go into a lot of hand wringing about what you learned or how you have grown as a person because, frankly, we know that those sentiments are mostly empty words that people think they need to say to show that they've changed their ways. Don't waste the time it will take me to read that, is my opinion of that sort of thing. Just give the facts and move on.
Frankly, I'd be surprised if anyone cares.