Boxing fans are urged to tune into PBS tonight at 9 pm for what will be a fascinating look at the life and troubled times of Jack Johnson, the heavyweight champion of the world nearly a century ago. The documentary will air in two two-hour parts, tonight and tomorrow. It was produced by Ken Burns, one of America's premier documentarians.  Johnson was the first black man to wear the heayweight crown and many parallels can be drawn between him and Mike Tyson. Johnson's title reign inspired ugly racism not only in many citizens but also from the United States government itself.  The film will feature plenty of footage of Johnson in the ring, although the ending of his bout against Tommy Burns was lost to the ages when local authorities ordered the camera to be shut off while Burns was being pummeled.