Stocks were lower at midday Wednesday after Fitch downgraded Greece’s credit rating, saying that Athens was likely to default even after it received a new round of bailout funds. The Dow Jones industrial average, the Nasdaq and the S&P 500 each slipped 0.2%. Volume was slightly lower compared with the same time Tuesday, down 1% on the Nasdaq and 3% on the NYSE. Wal-Mart (WMT) was down 2% in heavy volume, weighing on the Dow, after the world’s