Update URL to Groovy Docs

Closes spring-projectsgh-3715

Author: Riggs333

spring-boot-samples/spring-boot-sample-web-groovy-templates/README.adoc

@@ -1,5 +1,5 @@
 This sample application uses Spring Boot and
-http://beta.groovy-lang.org/docs/groovy-2.3.1/html/documentation/markup-template-engine.html[Groovy templates]
+http://docs.groovy-lang.org/latest/html/documentation/#_the_markuptemplateengine[Groovy templates]
 in the View layer. The templates for this app live in `classpath:/templates/`, which is
 the conventional location for Spring Boot. External configuration is available via
 ``spring.groovy.template.*''.