fix ledger.md

Author: Mr-Leshiy

docs/src/architecture/08_concepts/immutable_ledger/ledger.md

@@ -184,4 +184,7 @@ Block:
 <!-- OPTIONAL SECTIONS: see CIP-0001 > Document > Structure table -->
 
 [Catalyst Signed Document]: ./../signed_doc/spec.md
+[Catalyst Signed Document content]: ./../signed_doc/spec.md#signed-object-content
 [CBOR-deterministically-encoded]: https://datatracker.ietf.org/doc/html/rfc8949#name-deterministically-encoded-c
+[Brotli]: https://datatracker.ietf.org/doc/html/rfc7932
+[CBOR]: https://datatracker.ietf.org/doc/rfc8949/