input: Write a python function to split a string by spaces.
  assert split_string("hello world this is a test") == ["hello", "world", "this", "is", "a", "test"]
  assert split_string("split this string by spaces") == ["split", "this", "string", "by", "spaces"]
  assert split_string("one two three") == ["one", "two", "three"]
output: def split_string(s):
      return s.split()