WASHINGTON — Illinois Sen. Richard Durbin, chairman and ranking minority member of the Senate Judiciary Committee, sent a letter to an advocacy group for minor leaguers asking questions about baseball’s antitrust exemption.Durbin, a Democrat, and Charles Grassley, an Iowa Republican, sent the letter Tuesday to Harry Marino, executive director of Advocates for Minor Leaguers.The letter, first reported by The Washington Post, also was signed by Sen. Richard Blumenthal, D-Connecticut, and Sen. Mike Lee, R-Utah.The …