input: Write a python function to split a string into a list of words.
  assert split_into_words("Hello world") == ["Hello", "world"]
  assert split_into_words("Python is great") == ["Python", "is", "great"]
  assert split_into_words("Split this sentence into words") == ["Split", "this", "sentence", "into", "words"]
output: def split_into_words(s):
      return s.split()