[selectors] Minor issue with section 2 "Selectors Overview"

Hi,

The section 2 has a table with example of selectors. On the right hand side, there is the level in which the given selector was introduced.

I noticed a mistake:
 :indeterminate, :required, :optional
are all marked as “4”, that should be “3-UI/4” since those were introduced in CSS 3 UI: http://dev.w3.org/csswg/css-ui/ <http://dev.w3.org/csswg/css-ui/>

Benjamin