keycloak.plugins.keycloak-discord: correct license metadata

https://github.com/wadahiro/keycloak-discord/blob/v0.3.1/ejb/src/main/java/org/keycloak/social/discord/DiscordIdentityProvider.java

pkgs/servers/keycloak/keycloak-discord/default.nix

···
   meta = with lib; {
     homepage = "https://github.com/wadahiro/keycloak-discord";
     description = "Keycloak Social Login extension for Discord";
-    license = licenses.apsl20;
+    license = licenses.asl20;
     maintainers = with maintainers; [ mkg20001 ];
   };
 }