asciidoc: add minor workaround to add an empty line after code blocks

asciidoc: add minor workaround to add an empty line after code blocks

Insert an empty <simpara> in manpages after code blocks to force and
empty line.

The problem can be seen on the manpage for the git tutorial, where an
example command and the following paragraph is printed with no empty
line between them:

     First, note that you can get documentation for a command such as git
     log --graph with:

         $ man git-log
     It is a good idea to introduce yourself to git [...]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>