Fix json_update_videos()

    commit (1f5c95b) to transcript_utils.py changed the sig of download_youtube_subs from 
        def download_youtube_subs(youtube_subs, item, settings):
        to:
        def download_youtube_subs(youtube_id, video_descriptor, settings):

        hence taking a video id instead of a dict.

       This broke the download captions feature from the utilities page.