PHOTOGRAPHS - Hollywood, Sept. 10. Cinderella al midnight. Miriam Stevenson, the 21-year-old South Carolina college girl who won the title "Miss Universe" in July, packs away her purple swim suit as she prepared to leave for home and back to school. She is turning her back on the six-months, $250-a-week movie contract she won with the beauty title. The wholesome, country-reared blonde will return to classes at Lander College for girls at Greeenwood, S. C., nest monday. She told wewsmen: "I feel like a cinderella at midnight"