Tom Brokaw anchored NBC's "Nightly News" for 21 years. He traveled the world, covered 9/11, interviewed heads of state and followed Ronald Reagan's political career from beginning to end. His legacy, he said Wednesday, will probably involve none of that. Brokaw told an audience at the Los Angeles Public Library that his "single greatest contribution" will probably be his bestselling book, "The Greatest Generation." He talked about how on the flight to Los Angeles, two flight attendants gave him handwritten notes telling him how much the book had meant to him, how it had made them see their fathers in……