updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -277,6 +277,7 @@
     * [Prefix Sum](data_structures/arrays/prefix_sum.py)
     * [Product Sum](data_structures/arrays/product_sum.py)
     * [Rotate Array](data_structures/arrays/rotate_array.py)
+    * [Set Matrix Zeroes](data_structures/arrays/set_matrix_zeroes.py)
     * [Sparse Table](data_structures/arrays/sparse_table.py)
     * [Sudoku Solver](data_structures/arrays/sudoku_solver.py)
   * Binary Tree