nixos/gitea/mailer: update example configuration

Updated the email configuration example, as the previous example
did not work.

nixos/modules/services/misc/gitea.nix

···
             };
             mailer = {
               ENABLED = true;
-              MAILER_TYPE = "sendmail";
-              FROM = "[e-mail]";
-              SENDMAIL_PATH = "''${pkgs.system-sendmail}/bin/sendmail";
+              PROTOCOL = "smtp+starttls";
+              SMTP_ADDR = "smtp.example.org";
+              SMTP_PORT = "587";
+              FROM = "Gitea Service <[e-mail]>";
+              USER = "[e-mail]";
             };
             other = {
               SHOW_FOOTER_VERSION = false;