input: Write a python function to filter words that have exactly two letters.
  assert filter_two_letter_words(["hi", "hello", "it", "at", "a", "is"]) == ["hi", "it", "at", "is"]
  assert filter_two_letter_words(["be", "bee", "cat", "an", "on", "off"]) == ["be", "an", "on"]
  assert filter_two_letter_words(["go", "no", "yes", "ok", "so"]) == ["go", "no", "ok", "so"]
output: def filter_two_letter_words(words):
      return [word for word in words if len(word) == 2]