Trigger to UPDATE after UPDATE?

Use a BEFORE trigger instead, and set the updated column assigning a value to NEW.updated (this would be a simple assignment, not an UPDATE). This way you won't trigger additional UPDATEs.

Your trigger body will simply look like

SET NEW.updated = NOW()

I usually use AFTER triggers only for modifying other tables, BEFORE for modifying the new (or updated) row, or suppress DELETE.