rmilter: fix bindSocket.path

When /run/rmilter/ is wiped out the rmilter.sock is removed,
causing postfix to fail to contact rmilter.
Probably like described here: benoitc/gunicorn#1524 (comment)

nixos/modules/services/mail/rmilter.nix

@@ -89,7 +89,7 @@ in
 
       bindSocket.path = mkOption {
        type = types.str;
-       default = "/run/rmilter/rmilter.sock";
+       default = "/run/rmilter.sock";
        description = ''
           Path to Unix domain socket to listen on.
         '';