[css3-flexbox] Nit: One remaining s/flexbox/flex container/ needed in ED

Hi Tab,

I just noticed one remaining use of "flexbox" that wants to be replaced
with "flex container" in the flexbox ED:
  # 8. Alignment
  # After a flex container's contents have finished their flexing
  # and the dimensions of all flex items are finalized, they can
  # then be aligned within the flexbox.
http://dev.w3.org/csswg/css3-flexbox/#alignment

That last word there wants to be replaced with "flex container", I believe.

Thanks,
~Daniel