doc: fix minor typo

[blueyed]

No description provided.

[codecov-io]

Codecov Report

Merging #3554 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3554     +/-   ##
=========================================
- Coverage   97.94%   97.85%   -0.1%     
=========================================
  Files          43       43             
  Lines        8577     8576      -1     
  Branches     1384     1383      -1     
=========================================
- Hits         8401     8392      -9     
- Misses         72       80      +8     
  Partials      104      104

Impacted Files	Coverage Δ
aiohttp/web_app.py	98.87% <ø> (ø)	⬆️
aiohttp/locks.py	92.85% <0%> (-7.15%)	⬇️
aiohttp/web_fileresponse.py	96.55% <0%> (-1.73%)	⬇️
aiohttp/helpers.py	96.71% <0%> (-0.51%)	⬇️
aiohttp/streams.py	98.46% <0%> (-0.26%)	⬇️

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[asvetlov]

Thanks!