Fix outdated README for listers and improve formatting

While working on the Phabricator lister with @nahimilega, I found out that the README for swh-lister was quite outdated (https://forge.softwareheritage.org/!367).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.

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Migrated from D1461 (view on Phabricator)