Fix links to Spring-Framework reference docs.
Original pull request:
ifdef::backend-epub3[:front-cover-image: image:epub-cover.png[Front Cover,1050,1600]]
:spring-data-commons-docs: ../../../../spring-data-commons/src/main/asciidoc
:spring-data-r2dbc-javadoc: https://docs.spring.io/spring-data/r2dbc/docs/{version}/api
-:spring-framework-ref: https://docs.spring.io/spring/docs/{springVersion}/spring-framework-reference
+:spring-framework-ref: https://docs.spring.io/spring/docs/{springVersion}/reference/html
:reactiveStreamsJavadoc: https://www.reactive-streams.org/reactive-streams-{reactiveStreamsVersion}-javadoc
:example-root: ../../../src/test/java/org/springframework/data/r2dbc/documentation
:tabsize: 2
