input: Write a python function to convert a string to a list of words.
  assert string_to_list("This is a sentence.") == ["This", "is", "a", "sentence."]
  assert string_to_list("Hello world") == ["Hello", "world"]
  assert string_to_list("Python code") == ["Python", "code"]
output: def string_to_list(s):
      return s.split()