updating DIRECTORY.md

Author: Panquesito7

DIRECTORY.md

@@ -65,7 +65,7 @@
   * [Reverse A Linked List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/reverse_a_linked_list.cpp)
   * [Skip List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/skip_list.cpp)
   * [Sparse Table](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/sparse_table.cpp)
-  * [Stack](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack.h)
+  * [Stack](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack.hpp)
   * [Stack Using Array](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack_using_array.cpp)
   * [Stack Using Linked List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack_using_linked_list.cpp)
   * [Stack Using Queue](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack_using_queue.cpp)