input: Write a python function to replace a specific word in a sentence with "XXX".
  assert replace_word("It's a beautiful day", "beautiful") == "It's a XXX day"
  assert replace_word("This is a beautiful place", "beautiful") == "This is a XXX place"
  assert replace_word("Hello beautiful world", "beautiful") == "Hello XXX world"
output: def replace_word(sentence, word):
      return sentence.replace(word, "XXX")