[editorial] Fix reference to navigable (#602)

Author: antosart

index.bs

@@ -131,6 +131,7 @@ spec: HTML; urlPrefix: https://html.spec.whatwg.org/
     text: initializing a new Document object; url: initialise-the-document-object
     text: prepare the script element; url: prepare-the-script-element
     text: plugin; url: #plugin
+    text: navigable; url: #navigable
   type: attr-value
     for: link/rel; text: prefetch; url: link-type-prefetch
     for: link/rel; text: preconnect; url: link-type-preconnect