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:
 - crowdsourced
 language_creators:
 - other
-languages:
+language:
 - en
-licenses: [cc-by-4.0]
+license: [cc-by-4.0]
 multilinguality:
 - monolingual
 pretty_name: twitter covid19 tweets