TORONTO — Canada's main stock index fell more than 300 points Wednesday amid a broad-based decline, while U.S. markets also slid. Fitch Ratings cut the U.S. government’s credit rating from AAA to AA+ and that news was the focus for Wednesday, said Anish Chopra, managing director with Portfolio Management Corp. “It's really the Fitch news today that's dominating the market,” he said. Among Fitch’s concerns were the growing debt-to-GDP ratio, said Chopra, as well as rising fiscal deficits and higher debt repayment costs due to interest rate hikes. Investors are reacting by pulling back in one of the more risk-off…