input: Write a function to replace a given word in a sentence with another word. The function should take three parameters: the sentence, the old word to be replaced, and the new word to replace it with. The function should return the updated sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("Python is fun", "fun", "easy")=="Python is easy"
  assert replace_word("Let's go to the park", "park", "mall")=="Let's go to the mall"
  assert replace_word("She likes reading books", "books", "novels")=="She likes reading novels"
  assert replace_word("This is a test", "test", "example")=="This is a example"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)