Wrong arrow in CD enviroment (amsmath)

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

MathJax is not rendering diagrams correctly, I already filed an issue there:

https://github.com/mathjax/MathJax/issues/3445

Try render the following diagram:

```
\begin{CD}
B @> g >> A \\
@| @VV f V \\
B @= A
\end{CD}
```

#### Current behaviour

The horizontal line at the bottom is a single line, not a double line as it should be (see [documentation](https://www.latex-project.org/help/documentation/amsldoc.pdf)).

I know HackMD is only embedding MathJax, not developing it, but since KaTeX is also an option, which does not have this issue, I wanted to let the team know.