update governance contract ref doc:

https://docs.google.com/document/d/11iDJGpCDNrKmPQN8TOhmEqBlWZAOstH15j3Afc2xoQo/edit#heading=h.2qv8uxbx0yln

Author: chenquanyu

contracts/solidity/GovReward.sol

@@ -24,7 +24,7 @@ interface IGovernance {
         uint startHeight;
         address[] miners;
         uint preHeight;
-        uint draftId;
+        uint nextHeight;
         uint voteAmount;
     }