Fix `license` metadata

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

Thanks! 🙏

README.md

@@ -1,8 +1,8 @@
 ---
 pretty_name: Wikipedia Arabic dumps dataset.
-languages:
+language:
 - ar
-licenses:
+license:
 - unknown
 multilinguality:
 - monolingual