input: Write a function to replace a given string with another string in a sentence. The function should take three parameters: the sentence, the old word to be replaced, and the new word to replace the old word with. It should return the new sentence with the replacement made.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach" 
  assert replace_word("The sky is blue", "blue", "red")=="The sky is red"
  assert replace_word("", "word", "new_word")==""
  assert replace_word("Hello world", "not_present", "replacement")=="Hello world"
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)