Use proper element name for DocBook manpages.

The proper element name is "refentry", not "manpage".

This fixes #1134.
 `node.doctype` is invoking [this](https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor/document.rb#L567) method, isn’t it? If so, then it’s just returning instance variable when read for the second time – this costs almost nothing. It’s really not wise to make such “optimizations”, that are against code readability and language conventions, just for some theoretical 0.000001 % performance benefit. Never ever optimize without benchmarking.

“Premature optimization is the root of all evil!”