git-shortlog: Fix two formatting errors in asciidoc documentation

git-shortlog: Fix two formatting errors in asciidoc documentation

First use [verse] in the SYNOPSIS so that the line break actually
shows.

Secondly drop the quotes around '.mailmap' since this exposes
a bug in our toolchain (didn't bother enough yet to find out wether
it is asciidoc's fault or that of the XSL templates) that leads to
the dot not getting escaped correctly in the roff output and thereby
swallowing the line.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>