add missing certification line in block structure

rfc/0010_Duniter_Blockchain_Protocol_V12.md

@@ -980,6 +980,7 @@ Actives               | `IN` memberships (renewal)                        | Alwa
 Leavers               | `OUT` memberships                                 | Always
 Revoked               | Revocation documents                              | Always
 Excluded              | Exluded members' public key                       | Always
+Certifications        | Certification documents                           | Always
 Transactions          | A list of compact transactions                    | Always
 InnerHash             | The hash value of the block's inner content       | Always
 Nonce                 | An arbitrary nonce value                          | Always