Fixed mistype in 'Proxy Support' section of documentation

[cpwr]

Are there changes in behavior for the user?

No

Related issue number

None

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."

[asvetlov]

Thanks!

[codecov-io]

Codecov Report

Merging #2688 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2688   +/-   ##
=======================================
  Coverage   98.01%   98.01%           
=======================================
  Files          39       39           
  Lines        7245     7245           
  Branches     1258     1258           
=======================================
  Hits         7101     7101           
  Misses         44       44           
  Partials      100      100

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new