linked list must end with 'null'

We need to put 'null' to end of linked list. otherwise wen we want to get last item in the linked list, it will be undefined. And this situation inconsistent with object literal definition of linked list above.

Author: lumosmind

1-js/06-advanced-functions/01-recursion/article.md

@@ -459,6 +459,7 @@ let list = { value: 1 };
 list.next = { value: 2 };
 list.next.next = { value: 3 };
 list.next.next.next = { value: 4 };
+list.next.next.next.next = null;
 ```
 
 Here we can even more clearer see that there are multiple objects, each one has the `value` and `next` pointing to the neighbour. The `list` variable is the first object in the chain, so following `next` pointers from it we can reach any element.