input: Write a function to replace a given string in a sentence with another string. If the given string is not found in the sentence, return the original sentence. The function should take three parameters: the sentence, the old word to be replaced, and the new word to replace with. Return the modified sentence.
  
  assert replace_string("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_string("Coding is fun", "coding", "basketball") == "Coding is fun"
  assert replace_string("I went shopping today", "shopping", "fishing") == "I went fishing today"
  assert replace_string("", "anything", "nothing") == ""
  assert replace_string("Python is amazing", "amazing", "wonderful") == "Python is wonderful"
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)