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 word to be replaced, and the new word. It 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("Coding is fun", "fun", "exciting") == "Coding is exciting"
  assert replace_word("She sells sea shells", "sea", "land") == "She sells land shells"
  assert replace_word("", "walking", "running") == ""
  assert replace_word("Hello world", "world", "Universe") == "Hello Universe"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)