修复一个距离公式的typo

$$

\begin{aligned}

&d(x, y) &=& \sqrt{\left ( x_1 - y_1 \right )^2 + \left ( x_2 - y_2 \right )^2 + \cdot \cdot \cdot +\left ( x_n - y_n \right )^2}\\

&&=& \sqrt{\sum_{i = 1}^{n}(x_i - y_i)^2}

&\lVert\overrightarrow{AB}\rVert &=& \sqrt{\left ( x_{11} - x_{21} \right )^2 + \left ( x_{12} - x_{22} \right )^2 + \cdot \cdot \cdot +\left ( x_{1n} - x_{2n} \right )^2}\\

&&=& \sqrt{\sum_{i = 1}^{n}(x_{1i} - x_{2i})^2}

\end{aligned}

$$