Fix spec URL in Cross-Origin-Opener-Policy doc (#3193)

There is no the-cross-origin-opener-policy-header anchor in the HTML
spec; instead the best place for the MDN article to link to is the
the-headers anchor, which has the following text:

> A Document's cross-origin opener policy is derived from the
> `Cross-Origin-Opener-Policy` and the
> `Cross-Origin-Opener-Policy-Report-Only` HTTP response headers. These
> headers are structured headers whose value must be a token.

files/en-us/web/http/headers/cross-origin-opener-policy/index.html

@@ -75,7 +75,7 @@ <h2 id="Specifications">Specifications</h2>
  </thead>
  <tbody>
   <tr>
-   <td>{{SpecName('HTML WHATWG', '#the-cross-origin-opener-policy-header', 'Cross-Origin-Opener-Policy header')}}</td>
+   <td>{{SpecName('HTML WHATWG', '#the-headers', 'Cross-Origin-Opener-Policy header')}}</td>
   </tr>
  </tbody>
 </table>