update cdnjs sortable

add " at the end of src

Author: qnguyen12

README.md

@@ -289,7 +289,7 @@ https://jsfiddle.net/dede89/L54yu3L9/
 <script src="//cdnjs.cloudflare.com/ajax/libs/vue/2.5.2/vue.min.js"></script>
 
 <!-- CDNJS :: Sortable (https://cdnjs.com/) -->
-<script src="//cdn.jsdelivr.net/npm/[email protected]/Sortable.min.js></script>
+<script src="//cdn.jsdelivr.net/npm/[email protected]/Sortable.min.js"></script>
 
 <!-- CDNJS :: Vue.Draggable (https://cdnjs.com/) -->
 <script src="//cdnjs.cloudflare.com/ajax/libs/Vue.Draggable/2.15.0/vuedraggable.min.js"></script>