Update directory

Author: github-actions

DIRECTORY.md

@@ -607,6 +607,8 @@
         * [Math round](https://github.com/hoangtien2k3qx1/Java/blob/main/src/java/JavaCore/Math/Math_round.java)
         * [Math signum](https://github.com/hoangtien2k3qx1/Java/blob/main/src/java/JavaCore/Math/Math_signum.java)
         * [Math sqrt](https://github.com/hoangtien2k3qx1/Java/blob/main/src/java/JavaCore/Math/Math_sqrt.java)
+      * NestedClass
+        * [NestedClass](https://github.com/hoangtien2k3qx1/Java/blob/main/src/java/JavaCore/NestedClass/NestedClass.java)
       * [NguyenToDemo](https://github.com/hoangtien2k3qx1/Java/blob/main/src/java/JavaCore/NguyenToDemo.java)
       * OOP
         * JavaObjectClass