input: Write a function to sort a list of numbers in descending order.
  assert sort_descending([9, 2, 4, 7, 6])==[9,7,6,4,2]
  assert sort_descending([1, 3, 2, 4, 5])==[5,4,3,2,1]
  assert sort_descending([10, 20, 5, 15])==[20,15,10,5]
output: def sort_descending(lst):
      return sorted(lst, reverse=True)