input: Write a python function to sort an array in descending order.
  assert descending_order_sort([6, 4, 7, 2, 1, 3])==[7, 6, 4, 3, 2, 1]
  assert descending_order_sort([10, 20, 5, 15])==[20, 15, 10, 5]
  assert descending_order_sort([1, 2, 3, 4, 5])==[5, 4, 3, 2, 1]
output: def descending_order_sort(arr):
      return sorted(arr, reverse=True)