Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-9/README.md

@@ -9,3 +9,4 @@
 - [Result Class in Kotlin](https://www.baeldung.com/kotlin/result-class)
 - [Underscore Operator for Type Arguments in Kotlin](https://www.baeldung.com/kotlin/underscore-type-arguments)
 - [Using Enums With when() in Kotlin](https://www.baeldung.com/kotlin/enums-with-when)
+- [Calling Private Methods of a Class From Outside the Class](https://www.baeldung.com/kotlin/private-methods-outside-class)