Merge pull request #1543 from lumosmind/patch-12

linked list must end with 'null'

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.