update SF ref

Author: mnot

specs/_core.md

@@ -21,7 +21,7 @@ These RFCs collectively obsolete all preceding RFCs defining HTTP, including **R
 
 Also relevant is the Structured Field Values specification, which is used in a growing number of headers and trailers:
 
- * RFC 8941: [Structured Field Values for HTTP](/specs/rfc8941.html) ([test suite](https://github.com/httpwg/structured-field-tests))
+ * RFC 9651: [Structured Field Values for HTTP](/specs/rfc9651.html) ([test suite](https://github.com/httpwg/structured-field-tests))
 
 Protocols built "on top" of HTTP should follow Best Current Practice, as documented here: