input: Write a function to replace a given string with another string. The function should take in three arguments: a sentence, an old word, and a new word. It should return the new sentence after replacing all occurrences of the old word with the new word.
  assert replace_string("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_string("books and pens", "books", "notebooks")=="notebooks and pens"
  assert replace_string("dancing in the rain", "dancing", "singing")=="singing in the rain"
  assert replace_string("coding is fun", "fun", "awesome")=="coding is awesome"
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)