The short story "A Sound of Thunder" by Ray Bradbury. Supposedly the most re-published science fiction story of all time. You can read it at the link below.
http://www.lasalle.edu/~didio/courses/hon462/hon462_assets/sound_of_thunder.htm
(Ignore the film of the same name)
http://www.lasalle.edu/~didio/courses/hon462/hon462_assets/sound_of_thunder.htm
(Ignore the film of the same name)