input: Write a function to replace a given word in a sentence with a new word. The function should take three parameters: the sentence, the old word, and the new word. The function should return the new sentence after the replacement.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("Python is great", "great", "wonderful")=="Python is wonderful"
  assert replace_word("I like coding in Python", "coding", "programming")=="I like programming in Python"
  assert replace_word("Replace this word", "this", "that")=="Replace that word"
  assert replace_word("", "", "")==""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)