Fix outdated README for listers and improve formatting
While working on the Phabricator lister with @nahimilega, I found out that the README for swh-lister was quite outdated (https://forge.softwareheritage.org/!367).
That diff updates the README with up to date instructions for configuring / running a lister.
I also removed a deprecated configuration entry that is no more used in the current code.
Migrated from D1461 (view on Phabricator)