codec: README: updated GenVersion to 13

README.md

@@ -262,7 +262,7 @@ some caveats. See Encode documentation.
 
 ```go
 const CborStreamBytes byte = 0x5f ...
-const GenVersion = 12
+const GenVersion = 13
 var SelfExt = &extFailWrapper{}
 var GoRpc goRpc
 var MsgpackSpecRpc msgpackSpecRpc