Fix broken demo link in README.md
Fix broken demo link in README.md

After trying to see the demo for this project I noticed that the link
leads to a 404 page.

I found the updated link in [#957] and this commit updates the README to
link to the proper URL.

[#957]: https://github.com/Choices-js/Choices/issues/957

jshjohnson/Choices,jshjohnson/Choices,jshjohnson/Choices