Typo: \bar{\alpha}_t を $$ で囲む

- [該当箇所](https://shunk031.me/coloso-python-diffusion-models/materials/notebooks/noise-scheduler.html#:~:text=%E3%81%A7%E5%BE%97%E3%81%9F-,%5Cbar%7B%5Calpha%7D_t,-%E3%82%92%E5%85%83%E3%81%AB)