input: Write a python function to generate a list of words from a given string that start with a specific letter.
  assert generate_list("apple banana avocado berry apricot", "a") == ["apple", "avocado", "apricot"]
  assert generate_list("dog cat donkey elephant", "d") == ["dog", "donkey"]
  assert generate_list("fox frog fish ferret", "f") == ["fox", "frog", "fish", "ferret"]
output: def generate_list(s, letter):
      return [word for word in s.split() if word.startswith(letter)]