input: Write a function to replace a given word in a sentence with another word. The function should take three parameters: the original sentence, the old word to be replaced, and the new word to replace it with. The function should return the modified sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("The cat is sleeping", "sleeping", "playing")=="The cat is playing"
  assert replace_word("Python is fun", "fun", "awesome")=="Python is awesome"
  assert replace_word("Hello world", "world", "universe")=="Hello universe"
  assert replace_word("This is a test", "test", "demo")=="This is a demo"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)