docs: update DIRECTORY.md (TheAlgorithms#2827)

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DIRECTORY.md

@@ -325,6 +325,7 @@
   * [Addition Rule](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/addition_rule.cpp)
   * [Bayes Theorem](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/bayes_theorem.cpp)
   * [Binomial Dist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/binomial_dist.cpp)
+  * [Exponential Dist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/exponential_dist.cpp)
   * [Geometric Dist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/geometric_dist.cpp)
   * [Poisson Dist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/poisson_dist.cpp)
   * [Windowed Median](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/probability/windowed_median.cpp)