updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -277,6 +277,7 @@
     * [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)
   * Queues
     * [Circular Queue](data_structures/queues/circular_queue.py)