docs: add Ridge Regression entry to DIRECTORY.md

Author: Mahadev-05

DIRECTORY.md

@@ -609,6 +609,7 @@
   * [K Nearest Neighbours](machine_learning/k_nearest_neighbours.py)
   * [Linear Discriminant Analysis](machine_learning/linear_discriminant_analysis.py)
   * [Linear Regression](machine_learning/linear_regression.py)
+  * [Ridge Regression](machine_learning/ridge_regression.py)
   * Local Weighted Learning
     * [Local Weighted Learning](machine_learning/local_weighted_learning/local_weighted_learning.py)
   * [Logistic Regression](machine_learning/logistic_regression.py)