input: Write a python function to sort a list of integers in descending order.
  assert sort_descending([9, 2, 7, 1]) == [9, 7, 2, 1]
  assert sort_descending([1, 3, 5, 7, 9]) == [9, 7, 5, 3, 1]
  assert sort_descending([10, 20, 5, 3, 8, 15]) == [20, 15, 10, 8, 5, 3]
output: def sort_descending(lst):
      return sorted(lst, reverse=True)