nixos/molly-brown: fix description of certPath

`allowKeysForGroup` is no longer available so this drops

```
security.acme.certs."example.com".allowKeysForGroup = true;
```

line. `SupplementaryGroups` should be enough for
allowing access to certificates.

Changed files
-1
nixos
modules
services
web-servers
-1
nixos/modules/services/web-servers/molly-brown.nix
···
As an example:
<programlisting>
-
security.acme.certs."example.com".allowKeysForGroup = true;
systemd.services.molly-brown.serviceConfig.SupplementaryGroups =
[ config.security.acme.certs."example.com".group ];
</programlisting>