input: Write a python function to replace a word in a sentence with another word.
  assert replace_word("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_word("The cat is on the mat", "cat", "dog") == "The dog is on the mat"
  assert replace_word("Hello world", "world", "there") == "Hello there"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)