fix(bernoulli): fix format

$$

\begin{aligned}

F*n(z) &=\sum*{i=0}^{n-1}\sum*{m\ge 0}\dfrac{i^mz^m}{m!}\\

       &=\sum*{i=0}^{n-1}e^{iz}\\

F_n(z) &=\sum_{i=0}^{n-1}\sum_{m\ge 0}\dfrac{i^mz^m}{m!}\\

       &=\sum_{i=0}^{n-1}e^{iz}\\

       &=\dfrac{e^{nz} - 1}{e^z - 1}\\

       &=\dfrac{z}{e^z - 1}\cdot\dfrac{e^{nz} - 1}{z}

\end{aligned}