input: Write a function to convert a space-separated string into a list of words.
  assert string_to_list("Hello world this is a test") == ["Hello", "world", "this", "is", "a", "test"]
  assert string_to_list("Convert this string to a list") == ["Convert", "this", "string", "to", "a", "list"]
  assert string_to_list("Python code generation") == ["Python", "code", "generation"]
output: def string_to_list(s):
      return s.split()