[docs] [issue34764] Improve documentation example for using iter() with sentinel value

[ChrisRands]

Change by ChrisRands <chrisrands0 at gmail.com>:


----------
keywords: +patch
pull_requests: +10457
stage:  -> patch review

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34764>
_______________________________________