Editorial: Replace ref to obsoleted RFC7540 with ref to RFC9110 (#1106)

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>