Improve existing example checkbox.html

[valentin-p]

Improve an existing example.

I am fixing the inline html markup for "checked" in the checkbox example.

I noticed the issue in this site.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/checkbox

Before

After

Have a nice day.

[welcome]

💖 Thanks for opening this pull request! 💖
Here is a list of things that will help get it across the finish line:

• If this is a new or updated CSS interactive example, please ensure that you followed the CSS styleguide - If this is a new or updated JavaScript interactive example, please ensure that you followed the JavaScript styleguide - If your changes affects any of the steps in our contribution docs, please also make the relevant changes there.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[NiedziolkaMichal]

Thanks a lot, that's a good change.