Doc correction: Html.Attributes.checked

[pdavidow]

http://package.elm-lang.org/packages/elm-lang/html/2.0.0/Html-Attributes#checked also applies to type_ "radio"

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.