The president of the nation's largest teacher union won an unprecedented vote of confidence as a rules change allowing her and other officials to seek a third term won approval. Delegates to the National Education Assn.'s annual convention in Louisville, Ky., voted 4,218 to 1,607 to amend the 1.8-million-member union's constitution. The president, Mary Hatwood Futrell, is halfway through her second two-year term. She was expected to announce today that she will run again.