dnscrypt-proxy service: use mkEnableOption

nixos/modules/services/networking/dnscrypt-proxy.nix

···
 {
   options = {
     services.dnscrypt-proxy = {
-      enable = mkOption {
-        default = false;
-        type = types.bool;
-        description = ''
-          Enable dnscrypt-proxy. The proxy relays regular DNS queries to a
-          DNSCrypt enabled upstream resolver. The traffic between the
-          client and the upstream resolver is encrypted and authenticated,
-          which may mitigate the risk of MITM attacks and third-party
-          snooping (assuming the upstream is trustworthy).
-        '';
-      };
+      enable = mkEnableOption ''
+        Enable dnscrypt-proxy. The proxy relays regular DNS queries to a
+        DNSCrypt enabled upstream resolver. The traffic between the
+        client and the upstream resolver is encrypted and authenticated,
+        which may mitigate the risk of MITM attacks and third-party
+        snooping (assuming the upstream is trustworthy).
+      '';
       localAddress = mkOption {
         default = "127.0.0.1";
         type = types.string;