updating DIRECTORY.md

DIRECTORY.md

@@ -254,5 +254,6 @@
 
 ## Strings
   * [Brute Force String Searching](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/strings/brute_force_string_searching.cpp)
+  * [Horspool](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/strings/horspool.cpp)
   * [Knuth Morris Pratt](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/strings/knuth_morris_pratt.cpp)
   * [Rabin Karp](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/strings/rabin_karp.cpp)