clarify backend names in help text
clarify backend names in help text

pepijnve/asciidoctor,neontapir/asciidoctor,gjtorikian/asciidoctor,cuit-zhaxin/asciidoctor,benignbala/asciidoctor,glamouracademy/asciidoctor,benignbala/asciidoctor,gjtorikian/asciidoctor,anthonny/asciidoctor,anthonny/asciidoctor,glamouracademy/asciidoctor,asciidocfx/asciidoctor,cuit-zhaxin/asciidoctor,asciidocfx/asciidoctor,yukisan/asciidoctor,yukisan/asciidoctor,neontapir/asciidoctor,pepijnve/asciidoctor