[issue46224] doc: Fix bisect example using mutable function default

[Alex Waygood]

Change by Alex Waygood <Alex.Waygood at Gmail.com>:


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keywords: +patch
stage:  -> patch review
versions:  -Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46224>
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