input: Write a python function to replace a word in a sentence with another word.
  assert replace_word("Roses are red, violets are blue.", "blue", "red") == "Roses are red, violets are red."
  assert replace_word("I love apples and apples are sweet.", "apples", "oranges") == "I love oranges and oranges are sweet."
  assert replace_word("The cat sat on the mat.", "cat", "dog") == "The dog sat on the mat."
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)