added algorithm to DIRECTORY.md

Author: showmyth

DIRECTORY.md

@@ -174,6 +174,7 @@
     * Optimization
       * [Adam](https://github.com/TheAlgorithms/Rust/blob/master/src/machine_learning/optimization/adam.rs)
       * [Gradient Descent](https://github.com/TheAlgorithms/Rust/blob/master/src/machine_learning/optimization/gradient_descent.rs)
+      * [Momentum](https://github.com/TheAlgorithms/Rust/blob/master/src/machine_learning/optimization/momentum.rs)
   * Math
     * [Abs](https://github.com/TheAlgorithms/Rust/blob/master/src/math/abs.rs)
     * [Aliquot Sum](https://github.com/TheAlgorithms/Rust/blob/master/src/math/aliquot_sum.rs)