bad invocation of o.s.graph.backend.Setup in docker doc

it currently reads:

    $ java -cp /swh/app/swh-graph.jar       \
        org.softwareheritage.graph.backend.Setup /graph/compressed/g

but Setup needs two arguments instead of one, the first one being a full path to the .nodes.csv.gz file

(it's probably better to wait for !16 (closed) to be landed before fixing this)

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Migrated from T1933 (view on Phabricator)