This is embarrassing for them - not you. They're the one making fun of someone with a disability that they were unaware of, and in their surprise responded inappropriately. You'd know the feeling if you've done something like this to someone else. I still remember in 7th grade briefly making fun of one of my friends for having hand tremors, and I didn't understand at the time that it was an actual disability until another one of my friends pointed it out to me. I still remember the sense of embarrassment. You did the right thing by telling them that you stutter.