Emend demo code for registering treeprocessor

[andrewcarver]

I don't think having a |document| parameter is correct here, for the code-block passed to Asciidoctor::Extensions.register ...?? (It didn't work when I tried it, anyway--but I'm not sure why! :)

[mojavelinux]

You are correct. This is a remnant of an earlier design. The document is accessible via the registry, which is the optional first argument of the block.

Asciidoctor::Extensions.register do |registry|
  puts registry.document
end

[mojavelinux]

Thanks!