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 <hendeby@isy.liu.se>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -38,15 +38,16 @@ OPTIONS
 FILES
 -----
 
-.mailmap::
-	If this file exists, it will be used for mapping author email
-	addresses to a real author name. One mapping per line, first
-	the author name followed by the email address enclosed by
-	'<' and '>'. Use hash '#' for comments. Example:
-
-		# Keep alphabetized
-		Adam Morrow <adam@localhost.localdomain>
-		Eve Jones <eve@laptop.(none)>
+If the file `.mailmap` exists, it will be used for mapping author
+email addresses to a real author name. One mapping per line, first
+the author name followed by the email address enclosed by
+'<' and '>'. Use hash '#' for comments. Example:
+
+------------
+# Keep alphabetized
+Adam Morrow <adam@localhost.localdomain>
+Eve Jones <eve@laptop.(none)>
+------------
 
 Author
 ------