updating DIRECTORY.md

Author: mjk22071998

DIRECTORY.md

@@ -272,6 +272,7 @@
     * [Rotate To The Right](data_structures/linked_list/rotate_to_the_right.py)
     * [Singly Linked List](data_structures/linked_list/singly_linked_list.py)
     * [Skip List](data_structures/linked_list/skip_list.py)
+    * [Sorted Linked List](data_structures/linked_list/sorted_linked_list.py)
     * [Swap Nodes](data_structures/linked_list/swap_nodes.py)
   * Queue
     * [Circular Queue](data_structures/queue/circular_queue.py)