It pays to advertise. That is what Ziky Frahm, a Nigerian prince found when he wrote thiscolumn requesting a pen pal from the midwest. Truman Kephart, 437 Kern street, wrote the 15-year-old prince nearly a year ago. Truman received an answer last week. The prince explained that he had been to a meeting of the Nigerian princes in the interior of the country and hadn't been in a position to write because the meeting lasted six months. The letter was in a legible, but odd. script. The boy prince is a student in a Methodist high school