[css3-flexbox] Broken links to "#flexbox-item" instead of "#flex-items"

Hi www-style,

One Flexbox ED nit: right now, the spec (dated 20 March 2012) has a number
of internal links to "#flexbox-item", but those links don't work because
there's no element with that ID.

There is, however, an element with ID "flex-items", at the top of section
4 "Flexbox Items".  I think that's what these #flexbox-item links mean to
point to.

Looks like this needs a search-and-replace fixup to standardize on one or
the other, so that the links will work.

Thanks,
~Daniel