Fix normative reference to RFC7231

Author: lucacasonato

index.bs

@@ -140,7 +140,7 @@ Note: For example, in Node.js the global object does not implement {{EventTarget
 Requirements for default User-Agent value {#useragent-requirements}
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-The [=default `User-Agent` value=] is provided such that application code can reliably identify the runtime within which it is running. The value MUST be a string conforming to the <code class="idl"><a data-link-type="idl" href="https://datatracker.ietf.org/doc/html/rfc7231#section-5.5.3">`User-Agent`</a></code> construction in [[RFC7231]]:
+The [=default `User-Agent` value=] is provided such that application code can reliably identify the runtime within which it is running. The value MUST be a string conforming to the <code class="idl"><a data-link-type="idl" href="https://datatracker.ietf.org/doc/html/rfc7231#section-5.5.3">`User-Agent`</a></code> construction in [[!RFC7231]]:
 
 <pre>
   User-Agent      = product *( RWS ( product / comment ) )