docs: update DIRECTORY.md (#2771)

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DIRECTORY.md

@@ -399,6 +399,7 @@
 ## Strings
   * [Boyer Moore](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/boyer_moore.cpp)
   * [Brute Force String Searching](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/brute_force_string_searching.cpp)
+  * [Duval](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/duval.cpp)
   * [Horspool](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/horspool.cpp)
   * [Knuth Morris Pratt](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/knuth_morris_pratt.cpp)
   * [Manacher Algorithm](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/strings/manacher_algorithm.cpp)