input: def replace_string(sentence: str, old_word: str, new_word: str) -> str:
      """ Replace a given word in a sentence with another word.
      >>> replace_string("I love walking on the beach", "walking", "swimming")
      "I love swimming on the beach"
      >>> replace_string("The cat is sleeping on the mat", "cat", "dog")
      "The dog is sleeping on the mat"
      """
      
      new_sentence = sentence.replace(old_word, new_word)
      return new_sentence
output:     new_sentence = sentence.replace(old_word, new_word)
      return new_sentence