input: Write a function to create a Python program that takes in a positive integer and uses a while loop to output the numbers counting down from that integer to 1. The function should return the final message "Done!".
  python
  def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      return "Done!"
  
  assert countdown(6) == "Done!"
output: def countdown(n):
      while n > 0:
          print(n)
          n -= 1
      return "Done!"