input: Write a python function to split a string into a list of substrings using the specified separator and maxsplit parameters.
  assert split_string("apple,banana,cherry", ",", 1) == ["apple", "banana,cherry"]
  assert split_string("apple,banana,cherry", ",", 2) == ["apple", "banana", "cherry"]
  assert split_string("apple banana cherry", None, -1) == ["apple", "banana", "cherry"]
output: def split_string(s, sep=None, maxsplit=-1):
      return s.split(sep, maxsplit)