updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -92,6 +92,7 @@
   * [Count Sorted Vowel Strings](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/count_sorted_vowel_strings.rb)
   * [Fibonacci](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/fibonacci.rb)
   * [House Robber](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/house_robber.rb)
+  * [Knapsack](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/knapsack.rb)
   * [Ones And Zeros](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/ones_and_zeros.rb)
   * [Pascal Triangle Ii](https://github.com/TheAlgorithms/Ruby/blob/master/dynamic_programming/pascal_triangle_ii.rb)