Fix man page reference to --tools-tree-packages

[davide125]

This makes it match the man page.

[behrmann]

The fix should actually be the other way around and fix the man page. We do singular for the command line option and plural for the config file options. I think the idea is, that it's more natural to do --package --package bar, even though we accept --package foo,bar. I don't know whether this is a very good rationale, but at least it's mostly consistent.

[davide125]

Thanks for the context @behrmann, I've updated this to fix the manpage instead.