fix README - update link to 2.0 branch
fix README - update link to 2.0 branch

fix broken link to 2.0 branch, which is now maitenance
ysb33r/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,ysb33r/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin,asciidoctor/asciidoctor-gradle-plugin