input: 
  def replace_string(sentence: str, old_word: str, new_word: str) -> str:
      """Replace a given string within another string and return the modified sentence.
      >>> replace_string("I love walking on the beach", "walking", "swimming")
      'The new sentence is: \'I love swimming on the beach\''
      >>> replace_string("Python is fun", "Python", "Java")
      'The new sentence is: \'Java is fun\''
      >>> replace_string("Hello world", "Hello", "Goodbye")
      'The new sentence is: \'Goodbye world\''
      """
      new_sentence = sentence.replace(old_word, new_word)
      return "The new sentence is: '{}'".format(new_sentence)
  
output:     new_sentence = sentence.replace(old_word, new_word)
      return "The new sentence is: '{}'".format(new_sentence)