add missing part-title attribute to theming guide
add missing part-title attribute to theming guide

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