Fix incorrect example (examples/web_classview.py)

[prostomarkeloff]

What do these changes do?

Fix incorrect example.

Are there changes in behavior for the user?

It will work ;)

Related issue number

N\A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #4299 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4299      +/-   ##
==========================================
- Coverage   97.59%   97.58%   -0.02%     
==========================================
  Files          43       43              
  Lines        8853     8853              
  Branches     1385     1385              
==========================================
- Hits         8640     8639       -1     
- Misses         91       92       +1     
  Partials      122      122

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	97.72% <0%> (-0.57%)	⬇️

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[asvetlov]

thanks!