[issue34839] doctest: Change example under warnings section

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

I am posting this on core-mentorship. If you want to do this, post here and submit the PR within a day.  I will do the merge is no-one else does.

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versions: +Python 3.7

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