Say the mkdir the regeneration command needs

[zmaril]

Follow-up to #43, found by running its documented command on a fresh checkout of
merged main rather than by re-reading it.

The catalog is not committed and git does not track an empty directory, so
infact-packs/rust-std/api/ is absent after a clone — and infact catalog --output does not create the path it is given. The command in the README
therefore fails in exactly the situation it exists for:

$ infact catalog "$J/core.json" --package core --version "$V" \
      --output "infact-packs/rust-std/api/core-$V.json"
error: No such file or directory (os error 2)

which names neither the path nor the reason.

One line of fix and a sentence saying why it is there, so nobody deletes it as
noise. Verified after: the catalog builds, git status stays clean, and the
scan finds 30 slice::is_sorted and 11 all_unique on the CodeNet corpus.

The alternative — having --output create parent directories — is arguably the
better fix and would help every pack-building path, not just this one. Left
alone here because it changes CLI behaviour and this is a documentation bug.