fix(specs/withdrawal): Last of the Sherlock comments (#233)

Based on: https://github.com/sherlock-audit/2023-01-optimism-judging/blob/b7921698c1c537714c5f2021aa46ce0a1935ba39/1-specs-findings/1-processed/1-true/233.md#L1

specs/withdrawals.md

@@ -148,7 +148,7 @@ interface OptimismPortal {
 
     function proveWithdrawalTransaction(
         Types.WithdrawalTransaction memory _tx,
-        uint256 _l2BlockNumber,
+        uint256 _l2OutputIndex,
         Types.OutputRootProof calldata _outputRootProof,
         bytes[] calldata _withdrawalProof
     ) external;