Due to an incomplete command passed to the `HelpFormatter.format` method,
the text alignment in the `usage_prefix` variable was not the same as the
command output generated by click.
Those tests started to [[ https://jenkins.softwareheritage.org/job/DENV/job/tests/776/artifact/swh-loader-core.log | fail ]] since the recent growth of available loaders
as click wraps help lines to 80 columns.