input: Write a function to replace a given string with another string within a sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_word("Life is like a box of chocolates", "box", "bag") == "Life is like a bag of chocolates"
  assert replace_word("Coding is fun", "is", "was") == "Coding was fun"
  assert replace_word("The sky is blue", "is", "was") == "The sky was blue"
  assert replace_word("", "", "") == ""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)