fix the explanation of file name of docinfo1 attribute in user manual.

At [the section 7.2](http://asciidoctor.org/docs/user-manual/#docinfo-attributes-and-file-names) in [Asciidoctor User Manual](http://asciidoctor.org/docs/user-manual/), the table explains that I save a docinfo file as `<docname>.html` when using the attribute "docinfo1".
But, at proceeding texts, the item 2 of list explains "save the docinfo file as docinfo.html".

The table's text is a error.