input: Write a python function to reverse each word in a string.
  assert reverse_string("Hello world") == "olleH dlrow"
  assert reverse_string("Python is fun") == "nohtyP si nuf"
  assert reverse_string("Let's write some code") == "s'teL etirw emos edoc"
output: def reverse_string(s):
      return ' '.join(word[::-1] for word in s.split())