use hyphen in certain variable names in theme guide [skip ci]
use hyphen in certain variable names in theme guide [skip ci]

Hextremist/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,dsisnero/asciidoctor-pdf,Hextremist/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,dsisnero/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf