ddclient: Fix incorrectly capitalized systemd unit key

This avoids the following warning:

Apr 19 10:53:48 xen systemd[1]: [/nix/store/...-unit-ddclient.service/ddclient.service:19] Unknown lvalue 'type' in section 'Service'

As `Type=simple` is the default in systemd, the assignment to the
service type can be simply dropped.

nixos/modules/services/networking/ddclient.nix

@@ -127,8 +127,8 @@ in
       wantedBy = [ "multi-user.target" ];
       after = [ "network.target" ];
       serviceConfig = {
-        # This may change back to forking if too many problems occur:
-        type = "simple";
+        # Uncomment this if too many problems occur:
+        # Type = "forking";
         User = ddclientUser;
         Group = "nogroup"; #TODO get this to work
         PermissionsStartOnly = "true";