Fix typo in ClientWebSocketResponse.closed doc

[samueldg]

What do these changes do?

Fix a small typo in the client reference docs, for ClientWebSocketResponse.closed

"of" -> "on"

Are there changes in behavior for the user?

No.

Related issue number

No tracked issue.

[samueldg]

I didn't include a changelog entry, as the CONTRIBUTING file states these are for "non trivial" changes.

Let me know if you want me to add that!

[codecov-io]

Codecov Report

Merging #2425 into 2.3 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.3    #2425   +/-   ##
=======================================
  Coverage   97.23%   97.23%           
=======================================
  Files          39       39           
  Lines        8224     8224           
  Branches     1442     1442           
=======================================
  Hits         7997     7997           
  Misses         98       98           
  Partials      129      129

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new