Fix typo in `unkown_enum_as_default`

[Muzer]

I guess I'm the first person to use this piece of functionality, because in this function name, I noticed unknown is spelt as unkown!

The only reason I'm not making the change myself is that I'm not sure what your preferred way of avoiding breaking older clients would be. Suggest you could deprecate the current function and have it call the newly-renamed function which will now be the preferred entrypoint?