[CSS21] Minor detail float/formatting blocks

I think i missed the deadline, but i just noticed: 
http://www.w3.org/TR/2003/WD-CSS21-20030915/visuren.html#floats
# The margin box of an element in the normal flow that establishes a 
new
# block formatting context (such as a table, or ...

That 'table' should possibly read table-cell, but i think it makes 
more sense to update 
http://www.w3.org/TR/2003/WD-CSS21-20030915/visuren.html#q15
to include tables as well.

Also, i think it may be a good idea to change the heading
"9.4.1 Block formatting context"
to
"9.4.1 Block formatting contexts"
to make it more explicit that there are multiple.

 /Staffan