Fix `license` metadata

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

Thanks! 🙏

README.md

@@ -4,10 +4,10 @@ annotations_creators:
 - expert-generated
 language_creators:
 - Digital Umuganda
-languages:
+language:
 - en
 - rw
-licenses:
+license:
 - cc-by-4.0
 multilinguality:
 - multilingual