fixup anne's suggestion

Co-authored-by: Anne van Kesteren <[email protected]>

Author: mikewest

url.bs

@@ -3495,7 +3495,7 @@ null-or-<a>scalar value string</a> <var>base</var> (default null), and then runs
 </ol>
 </div>
 
-<div algorithm>
+<div algorithm="URL/extract an origin">
 <p>Objects implementing the {{URL}} interface's <a for="platform object">extract an origin</a> steps are
 to return <a>this</a>'s <a for=URL>URL</a>'s <a for=url>origin</a>. [[!HTML]]
 </div>