Fix `license` metadata

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

Thanks! 🙏

README.md

@@ -127,9 +127,9 @@ annotations_creators:
 - found
 language_creators:
 - found
-languages:
+language:
 - en
-licenses:
+license:
 - unknown
 multilinguality:
 - monolingual