reddit_sub_search: Use correct description field

The public description of a subreddit is now stored in field ``public_description``. The fields ``description`` and ``description_html`` are used for a different purpose and not appropriate for an instant answer result.

See also: https://github.com/reddit/reddit/commit/dbdf637d9fe66f99c290f041e3794e23a00bc06d and http://www.reddit.com/r/changelog/comments/tztot/reddit_change_subreddits_now_have_a_public/