services.cron.enable: Remove misleading default

Elsewhere we set it to "mkDefault (allFiles != [])" to the default has
no effect.

nixos/modules/services/scheduling/cron.nix

···
 
       enable = mkOption {
         type = types.bool;
-        default = true;
+        example = true;
         description = "Whether to enable the Vixie cron daemon.";
       };