declarative containers: improve example config

Container config example code mentions `postgresql` service, but the correct use of that service involves setting `system.stateVersion` option (as discovered in https://github.com/NixOS/nixpkgs/issues/30056).

The actual system state version is set randomly to 17.03 because I have no preferences here

nixos/modules/virtualisation/containers.nix

@@ -596,6 +596,8 @@ in
                   { config, pkgs, ... }:
                   { services.postgresql.enable = true;
                     services.postgresql.package = pkgs.postgresql96;
+                    
+                    system.stateVersion = "17.03";
                   };
               };
           }