[docs] [issue21485] remove unnecesary .flush() calls in the asyncio subprocess code example

[Berker Peksag]

Changes by Berker Peksag <berker.peksag at gmail.com>:


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nosy: +giampaolo.rodola, gvanrossum, haypo, pitrou, yselivanov

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21485>
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