Update README.md

[kning]

Postgres key is host not hostname see https://github.com/datafold/data-diff/blob/master/data_diff/databases/postgresql.py#L53

[erezsh]

Thank you for bringing it to our attention.

Can you please link to where it says that in the docs? (or anywhere official)

[kning]

It came up when I was running example locally, I got this error:

Looks it's just using psycopg2 library to open the connection, which is just host: https://www.psycopg.org/docs/module.html

[erezsh]

Thanks!