input: Write a function to replace a given word in a sentence with another specified word. If the given word is not found in the sentence, return the original sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("The sky is blue", "red", "green")=="The sky is blue"
  assert replace_word("", "word", "text")==""
  assert replace_word("This is a test", "", "example")=="This is a test"
  assert replace_word("Hello world", "world", "universe")=="Hello universe"
output: def replace_word(sentence, old_word, new_word):
      if old_word in sentence:
          return sentence.replace(old_word, new_word)
      return sentence