Fix a wrong errmsg in AlterRole()

[cca5507]

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

0001-Fix-a-wrong-errmsg-in-AlterRole.patch
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