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

[Serhiy Storchaka]

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.

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nosy: +serhiy.storchaka

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<http://bugs.python.org/issue11468>
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