fix(Icon): update typings for corner prop

[areading314]

See docs: https://react.semantic-ui.com/elements/icon/

[welcome]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-io]

Codecov Report

Merging #3393 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3393   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files         170      170           
  Lines        2809     2809           
=======================================
  Hits         2806     2806           
  Misses          3        3

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[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[layershifter]

Thanks much, missed this in #3315 👍

[levithomason]

Released in semantic-ui-react@0.85.0.