input: Write a function to replace a word in a string with another word.
  assert replace_word("Hello hello world!", "hello", "welcome") == "Hello welcome world!"
  assert replace_word("The quick brown fox jumps over the lazy dog", "fox", "cat") == "The quick brown cat jumps over the lazy dog"
  assert replace_word("I love programming", "love", "enjoy") == "I enjoy programming"
output: def replace_word(s, old, new):
      return s.replace(old, new)