Replace ref to obsoleted RFC7540 with ref to RFC9110

The spec mentions RFC7540 when it talks about content negotiation. RFC7540 has
been obsoleted by RFC9113, so reference should be updated.

Now, the prose is a bit clunky because it seems to suggest that RFC7540 defines
the `"Accept-Language"` HTTP header, whereas it does not. Rather than replacing
the reference with a reference to RFC9113, this update rather targets RFC9110,
where `"Accept-Language"` is defined.

index.html

@@ -3208,8 +3208,8 @@ <h2>
           The server that hosts the web application could attempt to
           predetermine the end-user's language by using <a href=
           "https://en.wikipedia.org/wiki/Geotargeting">geotargeting</a> or by
-          using content negotiation (e.g., using [[RFC7540]]'s
-          "`Accept-Language`" header, or even a custom HTTP header).
+          using content negotiation (e.g., using an HTTP "`Accept-Language`"
+          header [[RFC9110]], or even a custom HTTP header).
         </dd>
       </dl>
       <p>