Fix HTML version of the keyword table.

Really fix #17528.

Here is the resulting HTML output:

- https://dl.dropboxusercontent.com/u/107479221/reference.html#keywords

["Adjacent sibling selector" `+` was first introduced in CSS 2.1](https://developer.mozilla.org/en-US/docs/Web/CSS/Adjacent_sibling_selectors) and should be understood by many modern browsers.