nixos/doc/mailman: fix syntax in code block

Author: wolfgangwalther

nixos/modules/services/mail/mailman.md

@@ -19,8 +19,8 @@ For a basic configuration with Postfix as the MTA, the following settings are su
       local_recipient_maps = [ "hash:/var/lib/mailman/data/postfix_lmtp" ];
       relay_domains = [ "hash:/var/lib/mailman/data/postfix_domains" ];
       smtpd_tls_chain_files = [
-        config.security.acme.certs."lists.example.org".directory + "/full.pem"
-        config.security.acme.certs."lists.example.org".directory + "/key.pem"
+        (config.security.acme.certs."lists.example.org".directory + "/full.pem")
+        (config.security.acme.certs."lists.example.org".directory + "/key.pem")
       ];
     };
   };