fix README.md

Author: Mr-Leshiy

rust/signed_doc/README.md

@@ -27,5 +27,4 @@ Inspect document
 cargo run -p catalyst-signed-doc --example mk_signed_doc inspect signed_doc/doc.cose
 ```
 
-[COSE]: https://datatracker.ietf.org/doc/html/rfc9052
 [brotli]: https://datatracker.ietf.org/doc/html/rfc7932