Modify how the user connects in the custom config example

As the example uses a PostgreSQL superuser, we can safely use
`SET SESSION AUTHORIZATION` and leverage the existing connection to
the database, instead of opening up a new one.

Issue: [ch6752]

Author: Jonathan S. Katz

examples/custom-config/setup.sql

@@ -25,7 +25,7 @@ CREATE SCHEMA IF NOT EXISTS PGHA_USER;
 
 /* The following has been customized for the custom-config example */
 
-\c PGHA_DATABASE PGHA_USER;
+SET SESSION AUTHORIZATION PGHA_USER;
 
 CREATE TABLE custom_config_table (
 	KEY VARCHAR(30) PRIMARY KEY,