fix: correct indentation in mailserver configuration

The indentation in the mailserver.nix file was fixed to follow proper syntax guidelines. This was a minor change, but it improves code readability and adherence to stylistic standards.

systems/x86_64-linux/mx/mailserver.nix

@@ -226,7 +226,7 @@
         ];
       };
     };
-   };
+  };
 
   services.roundcube = {
     enable = true;