A spring bouquet for Linn Yann, the 12-year-old public school pupil who won the Chattanooga, Tenn., spelling bee the other day. Linn Yann? Indeed yes. Linn Yann is a Cambodian who came to the United States with her family in 1979 and had never been to school. It all goes to show what gumption and determination can achieve. Even knowing how to spell ''acronym'' - the winning word.