Fix "Usage" docstring of aiohttp.client.request

[wereii]

What do these changes do?

Fix wrong "Usage" of aiohttp.client.request's docstring (function returns context manager).

Are there changes in behavior for the user?

Better, reflecting docstring!

Related issue number

Fixes #4603

Checklist

• I think the code is well written (N/A)
• Unit tests for the changes exist (N/A)
• 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 #4604 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4604   +/-   ##
=======================================
  Coverage   97.57%   97.57%           
=======================================
  Files          43       43           
  Lines        8907     8907           
  Branches     1404     1404           
=======================================
  Hits         8691     8691           
  Misses         96       96           
  Partials      120      120           

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