Fix a wrong errmsg in AlterRole()

Hi,

```
postgres=# create group g1;
CREATE ROLE
postgres=# create role r1 in group g1;
CREATE ROLE
postgres=# set session authorization r1;
SET
postgres=> alter group g1 drop user r1;
ERROR:  permission denied to alter role
DETAIL:  Only roles with the ADMIN option on role "g1" may add members.
```

The errmsg is "add members", but we are doing a drop.

Here is a small patch to fix it.

--
Regards,
ChangAo Chen