nixos/syncthing: use `types.port`

nixos/modules/services/networking/syncthing.nix

···
                     };
 
                     localAnnouncePort = mkOption {
-                      type = types.nullOr types.int;
+                      type = types.nullOr types.port;
                       default = null;
                       description = ''
                         The port on which to listen and send IPv4 broadcast announcements to.