updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -473,6 +473,7 @@
   * [K Means Clustering Tensorflow](dynamic_programming/k_means_clustering_tensorflow.py)
   * [Knapsack](dynamic_programming/knapsack.py)
   * [Largest Divisible Subset](dynamic_programming/largest_divisible_subset.py)
+  * [Longest Arithmetic Subsequence](dynamic_programming/longest_arithmetic_subsequence.py)
   * [Longest Common Subsequence](dynamic_programming/longest_common_subsequence.py)
   * [Longest Common Substring](dynamic_programming/longest_common_substring.py)
   * [Longest Increasing Subsequence](dynamic_programming/longest_increasing_subsequence.py)