ANSWER - I suspect that whatever witch claimed he/she could do this was too busy waving their own dick around to give you an honest and coherent answer.
No is the answer. And it's a shame because I believe most people would benefit from the understanding of the opposite sex such a spell will bring. Magic cannot alter physiology on a short and temporary basis. File this spell away with the popular "glamours" that promise to change eye colour.
You have not shared why you are interested in such a spell. If you were merely curious tell your witch friend from me that they might want to think before spouting...
If you were hoping to use the spell as an evaluative step to see if you would be more comfortable in the body of a woman than you are in the body of a man then your doctor is a positive first port of call for such a discussion. Be assured there is no shame or negativity involved in wanting to investigate your options. I personally consider anybody capable of deciding what it is they want in life and going for it to be a brave and strong individual indeed. There is a blog here aimed at helping teens with transsexual issues and gender queries. Well written and worth a look.