Update examples for async write usage

[kxepal]

This follows #2483

[codecov-io]

Codecov Report

Merging #2770 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2770   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files          39       39           
  Lines        7340     7340           
  Branches     1285     1285           
=======================================
  Hits         7192     7192           
  Misses         47       47           
  Partials      101      101

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[asvetlov]

Nice!
Please commit

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new