Update prog.py

>>> prog.py --help
usage: prog.py [-h] [--sum] N [N ...]

Process some integers.

positional arguments:
  N           a sequence of integers for the program to process

optional arguments:
  -h, --help  show this help message and exit
  --sum       sum the integers (default: find the maximum)

>>> prog.py 1 2 3
3

>>> prog.py 1 2 3 --sum
6

>>> prog.py 1 2 3 -s
usage: prog.py [-h] [--sum] N [N ...]
prog.py: error: unrecognized arguments: -s