Fix typo in entry for Tax Data API #3263

[jcork4real]

Unkown is now Unknown

• [ X ] My submission is formatted according to the guidelines in the contributing guide
• [ X ] My addition is ordered alphabetically
• [ X ] My submission has a useful description
• [ X ] The description does not have more than 100 characters
• [ X ] The description does not end with punctuation
• [ X ] Each table column is padded with one space on either side
• [ X ] I have searched the repository for any relevant issues or pull requests
• [ X ] Any category I am creating has the minimum requirement of 3 items
• [ X ] All changes have been squashed into a single commit

[matheusfelipeog]

Hi @jcork4real

Thanks for your contribution ;)

We have some problems with Public APIs, so we won't be able to review/add this improvement right now.

Read this issue for more information, please: https://github.com/public-apis/public-apis/issues/3104