It is only a myth that Africans have an extra bone in their foot. An extra bone in the foot can occur during child development if two of the bones do not fuse together properly, but that would not be exclusive to any one race of humanity.
The average human foot has 26 bones, which is common for all humans. The big toe has two phalanges, and the other four toes each have three phalanges. In the middle of the foot are five metatarsal bones, each named metatarsal I-V. Directly connected to the metatarsal bones are four tarsal bones: the cuboid bone, the lateral cuneiform bone, the intermediate cuneiform bone and the medial cuneiform bone. Three more tarsal bones articulate with the tibia and fibula to form the ankle: the navicular bone, the talus bone and the calcaneus (also know as the heel bone).
Everyone has these bones in their feet. The myth that Africans have an extra bone in their foot or an extra muscle in their leg most likely comes from the perception that Africans are more athletic than any other human race. Especially in the USA, this is also a stereotype that is based in myth due to a majority of American professional athletes being of African descent.