[docs] [issue11468] Improve unittest basic example in the doc

Serhiy Storchaka added the comment: I think `self.assertRaises(TypeError, s.split, 2)` looks simpler. In any case two examples for assertRaises needed, simple inlined and more complicated use as context manager. ---------- nosy: +serhiy.storchaka _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue11468> _______________________________________