input: Write a function to replace a given string with another string. The function should take three parameters: the sentence in which to replace the word, the old word to be replaced, and the new word to replace with. The function should return the updated sentence.
  assert replace_string("I love walking on the beach", "walking", "swimming")== "I love swimming on the beach"
  assert replace_string("Python is fun", "fun", "amazing")== "Python is amazing"
  assert replace_string("I like coding", "coding", "programming")== "I like programming"
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)