In block mathJax, _ will be converted to em

[Vanessa219]

Describe the bug
In block mathJax, _ will be converted to em

To Reproduce

Marked String

$$ 
ALG.1  Gradient  Boost  \\\\
F_0(x) = arg \min_p \sum_{i=1}^N L(y_i, p)  \\\\
for\ m=1\ to\ M\ do:\\\\
\ \ \ \hat{y_i} =  -[\frac {\partial L(y_i,F(x_i))}{\partial F(x_i)}]_{F(x)}  \text{计算残差, 梯度下降的方向}
f_m = arg \min_p \sum_{i=1}^N L(y_i, F_{m-1}(x_i)+\eta q(x_i))  \text{基学习器h，优化f，计算步长} \\\\
F_m(x) = F_{m-1}(x) + \eta f_m(x)  \text{更新模型} \\\\
\text{end For}
$$

Expected behavior

see b3log/vditor#24 (comment) image

[UziTech]

You should be able to use a code block to prevent markdown parsing

```
$$ 
ALG.1  Gradient  Boost  \\\\
F_0(x) = arg \min_p \sum_{i=1}^N L(y_i, p)  \\\\
for\ m=1\ to\ M\ do:\\\\
\ \ \ \hat{y_i} =  -[\frac {\partial L(y_i,F(x_i))}{\partial F(x_i)}]_{F(x)}  \text{计算残差, 梯度下降的方向}
f_m = arg \min_p \sum_{i=1}^N L(y_i, F_{m-1}(x_i)+\eta q(x_i))  \text{基学习器h，优化f，计算步长} \\\\
F_m(x) = F_{m-1}(x) + \eta f_m(x)  \text{更新模型} \\\\
\text{end For}
$$
```

[UziTech]

marked does not understand mathJax

[UziTech]

see #722