Add qualifier to Disgaea's Etna tag

Posted under Tags

rename etna -> etna_(disgaea)

(can't make a request since the tag has more than 200 posts)

The current tag for Disgaea's Etna lacks any qualifier. Currently, there's a tag called etna_(kuzuyu) which is an original character of the same name. To futureproofing, I think her tag should be given a qualifier to avoid confusion.