[docs] [issue34839] doctest: Change example under warnings section

[Lysandros Nikolaou]

Lysandros Nikolaou <lisandrosnik at gmail.com> added the comment:

I would be happy to change this. I will submit a PR ASAP.

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nosy: +lys.nikolaou

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34839>
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