Fix all asciidoc warnings

[TheJavaGuy]

Asciidoc interprets strings like ${project-version} as attributes and then issues a WARN during the build that it will skip reference to a missing attribute. This PR removes subs attribute from code blocks so that Asciidoctor won't perform substitutions inside them.

Proof of all warnings gone:

Proof of warnings before this PR:

[majguo]

@all-contributors please add @TheJavaGuy for code

[allcontributors[bot]]

@majguo

I've put up a pull request to add @TheJavaGuy! :tada: