Being Japanese and all, she spells her name in Japanese characters. Phonetically, these characters are written TO-U-NO. When they're pronounced, though, the U isn't pronounced like a "U", instead the "O" sound is held for an extra beat. So "Touno" is closer to being written correctly and "Tohno" is closer to how it sounds. There are a few different ways of romanizing Japanese and this is one of the differences between systems. (A third way is to write it as "Tono", with a horizontal bar over the first "o". That's difficult to show on a computer or typewriter.)