docs(readme): fix link to demo page (#1939)

@waynevanson renamed the repo form `Sortable` to `sortablejs` so users don't get confused on which package to install

README.md

@@ -2,7 +2,7 @@
 
 Sortable is a JavaScript library for reorderable drag-and-drop lists.
 
-Demo: http://sortablejs.github.io/Sortable/
+Demo: https://sortablejs.github.io/sortablejs/
 
 [<img width="250px" src="https://raw.githubusercontent.com/SortableJS/Sortable/HEAD/st/saucelabs.svg?sanitize=true">](https://saucelabs.com/)