updating DIRECTORY.md

Author: Mayank-Creater

DIRECTORY.md

@@ -189,6 +189,7 @@
     * [Find Triplets With 0 Sum](data_structures/arrays/find_triplets_with_0_sum.py)
     * [Index 2D Array In 1D](data_structures/arrays/index_2d_array_in_1d.py)
     * [Kth Largest Element](data_structures/arrays/kth_largest_element.py)
+    * [Maximum Subarray Sum](data_structures/arrays/maximum_subarray_sum.py)
     * [Median Two Array](data_structures/arrays/median_two_array.py)
     * [Monotonic Array](data_structures/arrays/monotonic_array.py)
     * [Pairs With Given Sum](data_structures/arrays/pairs_with_given_sum.py)