AssetType Unknown path spelling

Summary

The path for an "Unknown" AssetType is misspelled "unkown", which would probably lead to a weird bug months from now if we left it in.

Relevant logs and/or screenshots

https://gitlab.idiap.ch/beat/beat.editor/blob/31bd792c5bd450167fbf24f39666d86576e43a0b/beat/editor/backend/assetmodel.py#L44