Updating the sache.json example file link to be an actual sache.json file

Currently the "Checkout a real live sache.json file" link points to a [sassmanifest.json](https://github.com/jhardy/omg-shadows/blob/master/sassmanifest.json) file.
It would be more helpful if it points to a [sache.json](https://github.com/jhardy/Sassy-Buttons/blob/master/sache.json) file.