input: Write a python function to sort a list in descending order.
  assert sort_descending([4, 2, 9, 1]) == [9, 4, 2, 1]
  assert sort_descending([10, 5, 3, 8, 2]) == [10, 8, 5, 3, 2]
  assert sort_descending([1, 2, 3, 4, 5]) == [5, 4, 3, 2, 1]
output: def sort_descending(lst):
      return sorted(lst, reverse=True)