nixos/tarsnap: update doc for printStats

after the change made in 15567e6d8e338f339e4fafada1362bdaa9612cb3

nixos/modules/services/backup/tarsnap.nix

···
                 description = ''
                   Print global archive statistics upon completion.
                   The output is available via
-                  <command>systemctl status tarsnap@archive-name</command>.
+                  <command>systemctl status tarsnap-archive-name</command>.
                 '';
               };