Fix docs markup

[asvetlov]

No description provided.

[codecov-io]

Codecov Report

Merging #4232 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4232      +/-   ##
==========================================
+ Coverage   97.55%   97.55%   +<.01%     
==========================================
  Files          43       43              
  Lines        8837     8841       +4     
  Branches     1383     1383              
==========================================
+ Hits         8621     8625       +4     
  Misses         93       93              
  Partials      123      123

Impacted Files	Coverage Δ
aiohttp/web_routedef.py	100% <0%> (ø)	⬆️
aiohttp/web_app.py	97.3% <0%> (ø)	⬆️
aiohttp/web_urldispatcher.py	99.09% <0%> (ø)	⬆️

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