refactor(email-options): rename selfprivacy.email.subdomain to domain

selfprivacy-module.nix

@@ -193,13 +193,13 @@ with lib;
     email = with lib; mkOption {
       type = types.attrsOf (
         types.submodule (
-          { name, ... }: {
+          { ... }: {
             options = {
-              subdomain = mkOption {
+              domain = mkOption {
                 type = with types; strMatching "[A-Za-z0-9][A-Za-z0-9\-]{0,61}[A-Za-z0-9]";
-                example = "myservice";
+                example = "myservice.example.net";
                 default = config.sp.domain;
-                description = "Email subdomain";
+                description = "Domain to send emails from";
               };
 
               systemdTargets = mkOption {