input: Write a function to replace a given word in a sentence with another word. The function should take the original sentence, the old word, and the new word as inputs and return the new sentence after the replacement.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("Python is amazing", "Python", "Java")=="Java is amazing"
  assert replace_word("The cat is on the roof", "cat", "dog")=="The dog is on the roof"
  assert replace_word("Hello world", "world", "universe")=="Hello universe"
  assert replace_word("", "anything", "nothing")==""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)