Fix wrong error message from pg_get_tablespace_ddl()

Hi,

I started testing pg_get_tablespace_ddl(). While tracing pg_get_tablespace_ddl_internal(), I noticed that this error report must be wrong:
```
/* User must have SELECT privilege on pg_tablespace. */
if (pg_class_aclcheck(TableSpaceRelationId, GetUserId(), ACL_SELECT) != ACLCHECK_OK)
{
ReleaseSysCache(tuple);
aclcheck_error(ACLCHECK_NO_PRIV, OBJECT_TABLESPACE, spcname);
}
```

The comment clearly says that SELECT privilege on pg_tablespace is required, but the error is reported against the target tablespace instead.

This is easy to reproduce:
```
evantest=# set allow_in_place_tablespaces = true;
SET
evantest=# create role r1;
CREATE ROLE
evantest=# create tablespace ts1 location '';
CREATE TABLESPACE
evantest=# revoke select on pg_tablespace from r1;
REVOKE
evantest=# set role r1;
SET
evantest=> select * from pg_get_tablespace_ddl('ts1');
ERROR: permission denied for tablespace ts1
```

Attached is a simple one-line fix. Attached is a simple one-line fix. I did not add a new test, as we usually try to avoid extending the test time for such a small fix. With the fix, the error message now looks like:
```
evantest=> select * from pg_get_tablespace_ddl('ts1');
ERROR: permission denied for table pg_tablespace
```

Oops, I was one of the reviewers of the original patch. Sorry for not finding this during review.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/