[docs] [issue35738] Update timeit documentation to reflect default repeat of three

[Henry Chen]

Change by Henry Chen <tahafut at gmail.com>:


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keywords: +patch, patch, patch
pull_requests: +11196, 11197, 11198
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35738>
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