prerequisites: Drop note about AsciiDoc ref, add xref to asciidoc-fedora
    
    I noticed the `prerequisites.adoc` page mentions that an AsciiDoc guide
    will be published soon and to check back later. Well, it is done now!
    :grinning: So, this commit drops that note and adds the right `xref` so
    someone can go find the AsciiDoc for Fedora module.
    
    Signed-off-by: Justin W. Flory (he/him) <git@jwf.io>