1. remove duplicate html_last_updated_fmt in doc/source/conf.py
the second one cause the new added one not take effect.
2. update html_last_updated_fmt in api-ref/source/conf.py
use the recommended '%Y-%m-%d %H:%M' data time format.
Change-Id: Ib37b8a05c88667b34438186c55b6eb3f64b64960