[selectors] incorrect reference to html

Selectors 4, about the :hover pseudo, say this:

 Host languages may define additional ways in which an element can match :hover.
 For example, [HTML5] defines a label element as matching :hover
 when its labelled control is hovered.

http://dev.w3.org/csswg/selectors/#the-hover-pseudo

However, html says the opposite:

 For the purposes of defining the :hover pseudo-class only,
 an HTML user agent must consider an element as being one that the user designates if it is […]
 an element that is the labeled control of a label element
 that is currently matching :hover.

https://html.spec.whatwg.org/multipage/scripting.html#selector-hover

Implementations match what html say.

We should change the text in selectors for to be:

 [HTML5] defines a labelled control as matching :hover
 when its label element is hovered.

 - Florian