Update prog.py

>>> prog.py 1 2 3
Namespace(accumulate=<built-in function max>, integers=[1, 2, 3])
3
Namespace(accumulate=<built-in function max>, integers=[1, 4, 7, 2])
7

>>> prog.py 1 2 3 --sum
Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3])
6
Namespace(accumulate=<built-in function max>, integers=[1, 4, 7, 2])
7