Fix build by referencing css22 instead of css2 (#816)

index.bs

@@ -2046,7 +2046,7 @@ BrowserResult = (
 
 Each [=/top-level traversable=] is associated with a single <dfn>client
 window</dfn> which represents a rectangular area containing the
-<a spec=css2>viewport</a> that will be used to render that [=/top-level
+<a spec=css22>viewport</a> that will be used to render that [=/top-level
 traversable=]'s [=active document=] when its [=visibility state=] is
 "<code>visible</code>", as well as any browser-specific user interface elements
 associated with displaying the traversable (e.g. any URL bar, toolbars, or OS