Follow-up from "Docs - Postgres Exporter - add advanced configuration options"

The following discussion from !21008 (merged) should be addressed:

The project's full name is PostgreSQL Server Exporter. WDYT about updating how our page refers to it?
This way, we address the project as it's called by its maintainers, and not using the repo name. Also, we'd use the proper full PostgreSQL name.

As it is now, with phrases like "To enable the postgres exporter...", it looks odd with lowercase "postgres".