Clarify mapping named parameters (#5204)

contracts/governance/extensions/GovernorCountingFractional.sol

@@ -43,7 +43,7 @@ abstract contract GovernorCountingFractional is Governor {
     /**
      * @dev Mapping from proposal ID to vote tallies for that proposal.
      */
-    mapping(uint256 => ProposalVote) private _proposalVotes;
+    mapping(uint256 proposalId => ProposalVote) private _proposalVotes;
 
     /**
      * @dev A fractional vote params uses more votes than are available for that user.