A.C.
If it bothers you that much, time a moment when you're calm and can say something in private, and just say "I heard you say call Sally "he" the other day, and I know it was completely accidental, but I felt I had to let you know that that really bothers me; I'm worried that hearing that, even accidentally, could affect her self-esteem either now or at a latter point. Thank you for listening to me and hearing my concern, and I appreciate the job you're doing with Sally and with all the kids--Sally says she loves having you as her teacher." Or something like that--make it just you, a parent, expressing a concern, and that you understand the teacher is working hard and doesn't mean to do it. I bet that bringing that to her attention will help her focus on avoid doing it again (knowing someone else noticed and mentions it can have that effect!).