Fix reference documentation url

In recent versions the trailing `/html5/` in the documentation url has been removed

Author: spikymonkey

README.adoc

@@ -15,7 +15,7 @@ Currently Cloud Foundry is supported as a target platform for service instance b
 
 === Getting Started
 
-See the https://spring.io/projects/spring-cloud-app-broker/[project site] and https://docs.spring.io/spring-cloud-app-broker/docs/current/reference/html5/[reference documentation] to get started building a service broker using this framework.
+See the https://spring.io/projects/spring-cloud-app-broker/[project site] and https://docs.spring.io/spring-cloud-app-broker/docs/current/reference/[reference documentation] to get started building a service broker using this framework.
 
 Take a look at this https://spring.io/blog/2019/05/30/introducing-spring-cloud-app-broker[introduction blog post] and to our https://github.com/spring-cloud-samples/spring-cloud-app-broker-samples[sample apps].