Maven site integration uses .adoc, not .ad

In site module in asciidoctor-maven-plugin, the only extension that
works is ".adoc". It's hardcoded here:

https://github.com/asciidoctor/asciidoctor-maven-plugin/blob/master/src/main/java/org/asciidoctor/maven/site/AsciidoctorSiteModule.java#L34

We could change that so that any of .ad|.adoc|.asciidoc could be used,
but for now, the documentation should reflect the reality of the current
release.