Update check-spelling CI action

You're using an alpha release. I released [check-spelling@v0.0.20](https://github.com/check-spelling/check-spelling/releases/tag/v0.0.20) a month ago and I'd like to migrate people to that release.

The general gist of this change is merging the content of:

https://github.com/check-spelling/spell-check-this/blob/744c66e2140fd8acaf5388efd0db3727d010d6e9/.github/workflows/spelling.yml

Note that I intend to disable the v0.0.20-alpha* releases shortly (people can choose to run them anyway, but this is not recommended).