[css-counter-styles] typo in current ED

Just noticed a minor typo at 
http://dev.w3.org/csswg/css-counter-styles-3/#counter-style-prefix (and 
in the LCWD from July).

# The prefix descripter specifies a <symbol> ...

s/descripter/descriptor/

And similarly in the following section 3.4 (suffix).

JK