Spring Reference the spring-framework-petclinic repository wich uses AspectJ

The canonical version of Spring Petclinic based on Spring Boot has removed the AspectJ support. The original fork version is still based on Spring Framework. It shows the use of AspectJ via the CallMonitoringAspect class and the <aop:aspectj-autoproxy> (main branch) and the @EnableAspectJAutoProxy (javaconfig branch). I let you check: https://github.com/search?q=repo%3Aspring-petclinic%2Fspring-framework-petclinic%20aspectj&type=code

In this Pull Request, I propose to reference the spring framework fork to keep in the documentation an example of AspectJ implementation without Spring Boot.