libvpx

usage.dox: fix encoder/decoder subpage list indent

use \li to separate the list items contained in conditionals. this
avoids the encode page becoming a sub-item of decode; likely a problem
in <1.8.3.
+ fix encoder conditional, spelling error
+ correct encode page name to match decode 'Encoding'

Change-Id: I67890f52bed8e708bad63fb8819a074e0beff2ca