Update DIRECTORY.md

Author: ZicsX

DIRECTORY.md

@@ -973,6 +973,7 @@
   * [Recursive Quick Sort](sorts/recursive_quick_sort.py)
   * [Selection Sort](sorts/selection_sort.py)
   * [Shell Sort](sorts/shell_sort.py)
+  * [Shrink Shell Sort](sorts/shrink_shell.py)
   * [Slowsort](sorts/slowsort.py)
   * [Stooge Sort](sorts/stooge_sort.py)
   * [Strand Sort](sorts/strand_sort.py)