Shares of FedEx plunged 21% Friday – the biggest one-day drop in its history – after the company warned late Thursday that a slowing economy will cause it to fall $500 million short of its revenue target. The company said it expects business conditions to further weaken in the current second quarter, which runs through November. While global revenue this quarter is likely to be flat compared to a year earlier, FedEx’s earnings are expected to plunge more than 40%. The warning sparked a broad sell-off in US stocks, and the Dow Transportation Index fell 5%, while shares of FedEx rival UPS closed about 5% lower. The 21% single-day loss for FedEx shares easily tops their 16% plunge the day of the 1987 stock market crash, and a 15% . . .