input: Write a function to replace a given string with another string. The function should take three parameters: the original sentence, the old word to be replaced, and the new word to replace it with. It should return the updated sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("Coding is fun", "fun", "exciting")=="Coding is exciting"
  assert replace_word("She goes to school", "goes", "walks")=="She walks to school"
  assert replace_word("The cat is on the table", "cat", "dog")=="The dog is on the table"
  assert replace_word("", "", "")==""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)