A handful of billionaires own the Knicks. A few private equity firms own slices of the rest of the league. Marques Colston thinks the fans should get a turn—and he’s selling them an in at $500 a share. Colston, the famous underdog receiver of the New Orleans Saints, and former mixed martial artist Nick Edwards have launched the Champion Fund, an investment fund built so that anyone—not just the Mark Cubans of the world—can buy into sports for as…