WASHINGTON, June 3 (AP)-- Pedro Albizu Campos, Puerto Rico Nationalist party leader who was convicted six years ago of conspiring to overthrow the United States Government, ended his prison sentence today and started a four-year term of probation. James Bennett, director of the Federal Bureau of Prisons, said the Harvard educated political leader, one of eight convicted in the plot in San Juan, was being released today from the Federal penitentiary at Atlanta.