Fix `license` metadata

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

Thanks! 🙏

README.md

@@ -1,9 +1,9 @@
 ---
 annotations_creators:
 - expert-generated
-languages:
+language:
 - pl
-licenses:
+license:
 - other
 multilinguality:
 - monolingual