Fix typo on CDDL file name

Author: jasagredo

src/network/node-to-node/txsubmission2/README.md

@@ -100,7 +100,7 @@ a transaction ID:
 
 ```cddl
 ;; txid.cddl
-{{#include txid.cddl}}
+{{#include txId.cddl}}
 ```
 
 A transaction as transmitted in `TxSubmission2` is a tag-encoded