Fix grammar for i18n advice in Processing Errors.

Co-authored-by: Ted Thibodeau Jr <[email protected]>

Author: msporny

index.html

@@ -1013,8 +1013,8 @@ <h3>Processing Errors</h3>
         </li>
         <li>
 All human-readable strings SHOULD be localized as described in Section 1 of
-[[[RFC9457]]], using [=language negotiation=] on the `Accept-Language` HTTP
-header field to select the most appropriate resources.
+[[[RFC9457]]], using [=language negotiation=] through the `Accept-Language`
+HTTP header field to select the most appropriate resources.
         </li>
       </ul>