fee/cose: fix stale [Decode] doc link in DecodeReader (FIL-569)

The DecodeReader doc still referenced [Decode], which #32 renamed to
DecodeEncrypt. Align the doc link with the current name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fyx4RZx2j88mTQyeavbs79

Author: claude

fee/cose/decode.go

@@ -194,7 +194,7 @@ func (e *DecodedEnvelope) EncStructure(externalAAD []byte) ([]byte, error) {
 // past the envelope, then from r, so only the (small) header is held in memory
 // and an arbitrarily large ciphertext can be streamed.
 //
-// It is the streaming, tag-dispatching counterpart to [Decode] and
+// It is the streaming, tag-dispatching counterpart to [DecodeEncrypt] and
 // [DecodeEncrypt0], and is as strict as they are: a byte-string protected
 // header, map headers without duplicate labels, a null detached body, and — for
 // tag 96 — at least one well-formed 3-element recipient. Any deviation returns