input: Write a python function to replace a word in a given string with another word.
  assert replace_word("the cat in the hat", "cat", "dog") == "the dog in the hat"
  assert replace_word("hello world", "world", "there") == "hello there"
  assert replace_word("python is fun", "python", "coding") == "coding is fun"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)