[css-writing-modes] small correction to unicode-bidi:isolate

http://www.w3.org/TR/css3-writing-modes/#unicode-bidi currently has a note
for the 'isolate' value:

> In Unicode 7.0 and beyond, this will correspond to adding an LRI (U+2066),
> for ‘direction: ltr’, or RLI (U+2067), for ‘direction: rtl’, at the start
of the element,
> and a PDI (U+2069) at the end of the element.

and a similar one for 'isolate-override'.

The last UTC meeting (Nov 2012) resolved to speed up the addition of
isolates; these characters will be available in Unicode 6.3 (sometime in
the summer of 2013). The 7.0 should be changed to 6.3 in both notes in
Wring Modes Level 3.

Aharon