Update readme to reflect SBLGNT license change

[githubyouser]

I see that the SBLGNT recently changed its license to CC BY 4.0, so the readme will need updated to reflect that fact.

[jtauber]

I just have to check no further changes have been made

The SBL GNT website itself is not yet up to date to reflect the change.

https://sblgnt.com/license/