input: Write a python function to split a string into a list of words.
  assert split_string("This is a sample string") == ["This", "is", "a", "sample", "string"]
  assert split_string("Hello World") == ["Hello", "World"]
  assert split_string("Python programming is fun") == ["Python", "programming", "is", "fun"]
output: def split_string(s):
      return s.split()