Editorial: use the right "active document" definition

The "currently focused area of a top-level traversable" algorithm was referencing the "active document" associated with a browsing context rather than the one associated with a navigable.

source

@@ -77843,7 +77843,7 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
    <li><p>If <var>traversable</var> does not have <span>system focus</span>, then return
    null.</p></li>
 
-   <li><p>Let <var>candidate</var> be <var>traversable</var>'s <span>active
+   <li><p>Let <var>candidate</var> be <var>traversable</var>'s <span data-x="nav-document">active
    document</span>.</p></li>
 
    <li><p>While <var>candidate</var>'s <span data-x="focused area of the document">focused