Fix broken link to integration graph reference docs in actuator api documentation

See [here](https://github.com/spring-projects/spring-boot/blob/6d8ba654cc1a325a1525d214988d4c0f910652c1/spring-boot-project/spring-boot-actuator-autoconfigure/src/main/asciidoc/endpoints/integrationgraph.adoc#response-structure).