[css3-flexbox] start & end aren't ideal antonyms for box-align/pack

Minor nit:

For the box-align and box-pack property values "start" and "end", it seems
to me that start/finish, begin/end are the logical pairings of words. (I'd
just use top/bottom/left/right, or if you really want to share values for
vertical & horizontal orientations, then front/back seem ok for box-pack --
can't think of any pair good for box-align).

--scott