https://github.com/spring-projects/spring-framework/blob/fd49c8f598a9ce33f7ce14ace31c86856819776b/gradle/docs.gradle#L150-L168
(fd49c8f is latest commit of master branch)
As you can see, I think it should be build/docs/ref-docs/html5 directory, not a build/asciidoc/html5 directory.
Comment From: sbrannen
Good catch.
This has been merged into master.
Thanks