fix typo of UNKOWN to UNKNOWN in add_library.py

[NickCrews]

Let me know if you want the commit message formatted differently or anything else.

I just noticed this by happy accident: I was grepping through pyarrow's codebase, but I accidentally searched for "unkown" instead of what I meant to search for, "unknown", and then found this file in the virtual environment. What a funny happy accident! And it was easy to find this repo and submit a PR so I thought why not :)