message: [mock]: fix fmt check
diff: @@ -66,7 +66,7 @@ impl TxPoolAsyncService for MockTxPoolService {
  None => Ok(self.pool.lock().unwrap().clone()),
  }
  }
  - async fn next_sequence_number(self, address: AccountAddress) -> Result<Option<u64>> {
  + async fn next_sequence_number(self, _address: AccountAddress) -> Result<Option<u64>> {
  todo!()
  }
  