markdown 数学公式左对齐

代码：

$$
\begin{aligned}
& 1*2 + 3 \\
& = 2 + 3 \\
& = 5
\end{aligned}
$$

显示：
$$\begin{array}{rl}& 1\ast 2+3\\ & =2+3\\ & =5\end{array}$$