Update README.md

Author: rcalago

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

@@ -3,3 +3,4 @@
 - [Check if a String Is a Palindrome in Scala](https://www.baeldung.com/scala/string-palindrome-test)
 - [Scala’s Destructuring with the @ Operator](https://www.baeldung.com/scala/destructuring-operator)
 - [Check if Two Strings Are Isomorphic in Scala](https://www.baeldung.com/scala/string-isomorphism)
+- [Find Two’s Complement of a Number in Scala](https://www.baeldung.com/scala/compute-twos-complement)