list_comments: inconsistency in order naming (by_update vs by_last_update)

Hivemind: list_comments

supported_order_list = ['by_cashout_time', 'by_permlink', 'by_root', 'by_parent', 'by_update', 'by_author_last_update']

Hive: list_comments

'by_cashout_time', 'by_permlink', 'by_root', 'by_parent', 'by_last_update', 'by_author_last_update'