input: Write a function to replace a given string with another string. The function should take three parameters: the original sentence, the word to be replaced, and the new word. Return the sentence with the replaced word.
  assert sentence_replace("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert sentence_replace("The quick brown fox", "brown", "blue")=="The quick blue fox"
  assert sentence_replace("Coding is fun", "fun", "challenging")=="Coding is challenging"
  assert sentence_replace("Hello world", "world", "Python")=="Hello Python"
  assert sentence_replace("", "anything", "nothing")==""
output: def sentence_replace(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)