admin_audit file name does not use log_dir value

In the description for the `admin_audit_file` configuration parameter (link:
https://mariadb.com/kb/en/mariadb-maxscale-2302-mariadb-maxscale-configuration-guide/#admin_audit_file) it says the default is "admin_audit.csv in [logdir]" where "[logdir]" is linking to https://mariadb.com/kb/en/mariadb-maxscale-2302-mariadb-maxscale-configuration-guide/#logdir .

Configuring `logdir` to point to /var/lib/maxscale/log, the value of `admin_audit_file` is `/var/log/maxscale/admin_audit.csv` (which uses the default value of `logdir`, not the overridden version).