[csswg-drafts] Pull Request: [cssom-1] Add missing extract class on dashed_attribute example

SaschaNaz has just submitted a new pull request for https://github.com/w3c/csswg-drafts:

== [cssom-1] Add missing extract class on dashed_attribute example ==
https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_dashed_attribute

```WebIDL
partial interface CSSStyleDeclaration {
  [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _dashed_attribute;
};
```

This IDL snippet does not have `extract` class attached and thus collected on IDL index, whereas other _camel_cased_attribute and _webkit_cased_attribute examples are not collected.

See https://github.com/w3c/csswg-drafts/pull/1389