input: Write a function to sort a list of numbers in descending order using bubble sort.
  assert sortDescending([6, 2, 12, 5]) == [12, 6, 5, 2]
  assert sortDescending([10, 8, 9, 7]) == [10, 9, 8, 7]
  assert sortDescending([1, 2, 3, 4]) == [4, 3, 2, 1]
output: def sortDescending(arr):
      n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] < arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr