shortlog manpage documentation: work around asciidoc markup issues

shortlog manpage documentation: work around asciidoc markup issues

We wanted to have a list in which one (and the sole, as it happen to
be) item in it is ".mailmap", but do not seem to be able to convince
AsciiDoc to format it correctly for manpages.  Reformat it into a
paragraph that describes the said file to work around the issue.

Signed-off-by: Gustaf Hendeby <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>