Fix freemarker documentation link

Closes gh-14046

Author: frohwerk

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -1979,7 +1979,7 @@ and JSPs. Many other templating engines also ship their own Spring MVC integrati
 
 Spring Boot includes auto-configuration support for the following templating engines:
 
- * http://freemarker.org/docs/[FreeMarker]
+ * https://freemarker.apache.org/docs/[FreeMarker]
  * http://docs.groovy-lang.org/docs/next/html/documentation/template-engines.html#_the_markuptemplateengine[Groovy]
  * http://www.thymeleaf.org[Thymeleaf]
  * http://mustache.github.io/[Mustache]