modules/postfix: fix manpage number in option description (#440866)

nixos/modules/services/mail/postfix.nix

···
         default = "";
         type = lib.types.lines;
         description = ''
-          Entries for the transport map, cf. man-page {manpage}`transport(8)`.
+          Entries for the transport map, cf. man-page {manpage}`transport(5)`.
         '';
       };