I saw that you ordered red stars. They are sex links. Which are crosses that can be sexed by looking at the color of the chicks at hatch. They would not have vent sexed your chicks since there is no need with this type of hybrid breed, which may be why this was missed at the hatchery. Red stars are egg laying machines and are used commercially for brown egg production. If it is not bothered by the leg leave it be. You could try to remove the leg if it bothers you. Banding it tightly like you would a male calfs testicles would cause the extra leg to fall off in a few days with no loss of blood to the chick. Personally I would leave it alone, but it is an option. Good luck with her.