Correct license information of validator data

Author: FranklinYu

_data/validator-libraries.yml

@@ -4,7 +4,7 @@
     - name: Json.NET Schema
       url: https://www.newtonsoft.com/jsonschema
       draft: [3, 4, 6]
-      license: "MIT"
+      license: "AGPL-3.0-only"
     - name: Manatee.Json
       url: https://github.com/gregsdennis/Manatee.Json
       draft: [4, 6, 7]