docs: Javalin sample README claims AMS authorization is an unimplemented TODO, but it is fully implemented

[arnoweiss]

ams-javalin-shopping/README.md currently claims authorization isn't implemented [1]. Instead of (as claimed) a stub, the authorization is implemented in AuthHandler.java [2].

[1] https://github.com/SAP-samples/ams-samples-java/blob/main/ams-javalin-shopping/README.md#L114-L123
[2] https://github.com/SAP-samples/ams-samples-java/blob/main/ams-javalin-shopping/src/main/java/com/sap/cloud/security/ams/samples/auth/AuthHandler.java