[css2] typo in example

[babatakao]

https://drafts.csswg.org/css2/generate.html#undisplayed-counters

This example uses pseudo-element with two colons, which is not defined by CSS2.

h1::before {
    content: normal;
    counter-increment: heading;
}

[tabatkins]

That just means that CSS2 needs to have the double-colon syntax backported to it. (Or we need to remove this section, as it's replaced by the Lists spec.) We don't promote the single-colon syntax for pseudo-elements; it's a legacy mistake that we have to support, but we downplay it and don't use it in examples.