input: Write a function to find words that start with a given string and are longer than 2 characters.
  assert completion("th") == ["the", "that", "this", "those", "thing", "think", "thought"]
  assert completion("thi") == ["this", "thing", "think", "thought"]
  assert completion("tho") == ["those", "thought"]
output: def completion(prefix):
      words = ["the", "that", "this", "those", "thing", "think", "thought"]
      return [word for word in words if word.startswith(prefix) and len(word) > 2]