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

mentioned in commit df2279ef

mentioned in merge request !43 (merged)

mentioned in commit 49dddc80

closed

mentioned in commit 12e8ad12

mentioned in commit 3bafb87b

mentioned in commit dd1906da

mentioned in commit 849d0867