Even though this question has the ability to create a somewhat negative tone, you can still answer it in a positive way.
"When I first started my career, I wasn't as focused on a speciality and ended up spending a lot of time working with students who had more low-incidence disabilities. I wish I had been introduced to more specific work, with individuals affected by Autism, sooner in my career."
"I would've focused on special education much earlier. When I first started my education, I only knew that I wanted to teach, but I didn't know what population was most suited to me yet. Now that I know that special education is what I enjoy the most, if I could go back, I'd specialize in that from the beginning."