fix error message for secret key parsing.

keys.go

@@ -34,7 +34,7 @@ func SecretKeyFromHex(skh string) (SecretKey, error) {
 	if len(skh) < 64 {
 		skh = strings.Repeat("0", 64-len(skh)) + skh
 	} else if len(skh) > 64 {
-		return id, fmt.Errorf("pubkey should be at most 64-char hex, got '%s'", skh)
+		return id, fmt.Errorf("secret key should be at most 64-char hex, got '%s'", skh)
 	}
 
 	if _, err := hex.Decode(id[:], unsafe.Slice(unsafe.StringData(skh), 64)); err != nil {