[css-nav-1] Link to the right place for "inert"

inert as a concept exists in the HTML spec, but it is not accessible to
authors, just some internal logic. inert as an attribute is (for now)
only defined in a WICG explainer / polyfill. Since we're talking about
the attribute, that is where we need to link.

Author: frivoal

css-nav-1/Overview.bs

@@ -1081,7 +1081,7 @@ run the following steps:
     authors should use make these elements unreachable to spatial navigation
     using the same best practices as for making elements unreachable to sequential navigation,
     such as using <code>tab-index="-1"</code>
-    or the <a href="https://html.spec.whatwg.org/multipage/interaction.html#inert"><code>inert</code></a> attribute.
+    or the <a href="https://github.com/WICG/inert"><code>inert</code></a> attribute.
     </div>
 
 6. Return <var>visibles</var>.