History doc page clarification on naming

Further to my previous patch to update the licence preamble (https://www.postgresql.org/message-id/CA%2BOCxoy1Z05qDb1BfZZaVzT%3D6BrEW6u3wxprBkc7pewThJMoCA%40mail.gmail.com), the attached patch updates the history doc page to properly reflect the use of the name "Postgres".

Text by Jonathan Katz, I just turned it into a patch.

This should be backpatched to all supported branches.