Docs: The current credential example on the `read_postgres` page doesn't work

[talagluck]

Description

We should update this to use a more typical connection string.

[greyscaled]

The following works both on GlareDB CLI (0.9.2) and QA with latest main

select * from read_postgres('host=pg.demo.glaredb.com port=5432 user=demo password=demo dbname=postgres', 'public', 'customer');