Change start_url - Document URL check from within-scope to same-origin.

This appears to have been a mistake.

index.html

@@ -323,7 +323,7 @@ <h2>
           </ol>
         </li>
         <li>Otherwise, if |manifest|["{{WebAppManifest/start_url}}"] is not
-        present, or not <a>within scope</a> of the {{Document}}'s
+        present, or not <a>same origin</a> as the {{Document}}'s
         {{Document/URL}}, consider the document not <a>installable</a>.
         </li>
         <li>Otherwise, the {{Document}} MAY be considered <a>installable</a>