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: http://git.io/ghiLbA

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.
Merge pull request #303 from grdryn/patch-1

Maven site integration uses .adoc, not .ad