[docs] [issue16077] fix code example in docs for built-in reduce()

Serhiy Storchaka added the comment: Indeed. Will probably want to add an example with 1-element sequence for functools.reduce in Doc/howto/functional.rst. ---------- nosy: +storchaka _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16077> _______________________________________