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