Update RFC comment

Responds to:
BlockstreamResearch#278 (comment)

Author: jesseposner

src/modules/frost/keygen_impl.h

@@ -125,7 +125,7 @@ int secp256k1_frost_shares_trusted_gen(const secp256k1_context *ctx, secp256k1_f
     secp256k1_sha256_finalize(&sha, polygen);
 
     /* Derive shares */
-    /* See draft-irtf-cfrg-frost-08#appendix-C.1 */
+    /* See RFC 9591, appendix C.1 */
     for (i = 0; i < n_participants; i++) {
         secp256k1_scalar share_i, idx;