input: Write a python function to extract words that start with a given letter from a list.
  assert extract_words(["apple", "banana", "apricot", "cherry", "avocado"], "a") == ["apple", "apricot", "avocado"]
  assert extract_words(["grape", "orange", "kiwi", "pear", "pineapple"], "p") == ["pear", "pineapple"]
  assert extract_words(["blueberry", "blackberry", "banana", "boysenberry", "blackcurrant"], "b") == ["blueberry", "blackberry", "banana", "boysenberry", "blackcurrant"]
output: def extract_words(word_list, letter):
      return [word for word in word_list if word.startswith(letter)]