input: Write a python function to filter words that do not contain a specific letter.
  assert filter_words(["apple", "banana", "cherry", "date", "fig"], "a") == ["cherry", "fig"]
  assert filter_words(["elephant", "tiger", "lion", "giraffe", "monkey"], "e") == ["tiger", "lion", "giraffe"]
  assert filter_words(["house", "car", "bicycle", "train", "plane"], "r") == ["house", "bicycle", "plane"]
output: def filter_words(words, letter):
      return [word for word in words if letter not in word]