Update the install doc for PostgreSQL 15

Fixes #12768

docs/installation/1-postgresql.md

@@ -55,6 +55,9 @@ Within the shell, enter the following commands to create the database and user (
 CREATE DATABASE netbox;
 CREATE USER netbox WITH PASSWORD 'J5brHrAXFLQSif0K';
 ALTER DATABASE netbox OWNER TO netbox;
+-- the next two commands are needed on PostgreSQL 15 and later
+\connect netbox;
+GRANT CREATE ON SCHEMA public TO netbox;
 ```
 
 !!! danger "Use a strong password"