input: Write a python function to split a string into a list using comma as a delimiter.
  assert split_string("apple,banana,cherry")==["apple","banana","cherry"]
  assert split_string("1,2,3,4,5")==["1","2","3","4","5"]
  assert split_string("dog,cat,bird")==["dog","cat","bird"]
output: def split_string(s):
      return s.split(',')