docs: Update README example to use stable key prop

Author: Claudéric Demers

README.md

@@ -60,7 +60,7 @@ const SortableList = SortableContainer(({items}) => {
   return (
     <ul>
       {items.map((value, index) => (
-        <SortableItem key={`item-${index}`} index={index} value={value} />
+        <SortableItem key={`item-${value}`} index={index} value={value} />
       ))}
     </ul>
   );