docs: fix edge case in rational number conversion `float(a//b)` (#56772)
Fixes #56726 added the changes that were suggested. fixing the mistake.
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Co-authored-by: Max Horn <max@quendi.de>
Co-authored-by: Chengyu Han <git@wo-class.cn>
(cherry picked from commit fc9e7c4a6a36d7d54ae7dd8b89ac14a8334a59c4)