Fix `license` metadata

We recently updated the datasets metadata for consistency with other repo types (models & spaces)

Thanks! 🙏

README.md

@@ -3,9 +3,9 @@ annotations_creators:
 - none
 language_creators:
 - unknown
-languages:
+language:
 - unknown
-licenses:
+license:
 - cc-by-nc-sa-4.0
 multilinguality:
 - unknown