Incorrect log message in store/fs.go

[iain-macdonald]

I believe the log message on line 244 of store/fs.go is incorrect. Instead of saying "rootnode.Lookup: uknown node type detected" it should say "refnode.Lookup: uknown node type detected" as it's a method on refnode. I opened a PR to fix this, but I can't request a reviewer as I don't have write access to the containerd/stargz-snapshotter repository.

[ktock]

Fixed in #1595