nixos/prometheus-wireguard-exporter: fix broken options

This is apparently a breaking change in a patch-level release[1] where
it's now necessary to specify values for each CLI argument.

[1] https://github.com/MindFlavor/prometheus_wireguard_exporter/releases/tag/3.6.1

nixos/modules/services/monitoring/prometheus/exporters/wireguard.nix

@@ -57,9 +57,9 @@ in {
         ${pkgs.prometheus-wireguard-exporter}/bin/prometheus_wireguard_exporter \
           -p ${toString cfg.port} \
           -l ${cfg.listenAddress} \
-          ${optionalString cfg.verbose "-v"} \
-          ${optionalString cfg.singleSubnetPerField "-s"} \
-          ${optionalString cfg.withRemoteIp "-r"} \
+          ${optionalString cfg.verbose "-v true"} \
+          ${optionalString cfg.singleSubnetPerField "-s true"} \
+          ${optionalString cfg.withRemoteIp "-r true"} \
           ${optionalString (cfg.wireguardConfig != null) "-n ${escapeShellArg cfg.wireguardConfig}"}
       '';
       RestrictAddressFamilies = [