nixos/matomo: Fix typo in assertion message (#365931)

This typo is confusing because it seems like the assertion requires nginx to be non-null (old text: "either nginx or nginx.webServerUser is mandatory").

nixos/modules/services/web-apps/matomo.nix

···
 
     assertions = [ {
         assertion = cfg.nginx != null || cfg.webServerUser != null;
-        message = "Either services.matomo.nginx or services.matomo.nginx.webServerUser is mandatory";
+        message = "Either services.matomo.nginx or services.matomo.webServerUser is mandatory";
     }];
 
     users.users.${user} = {