Dollar ($) symbol not escaped in inline MathJax

### 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

### Read our Official tutorial

It's on https://hackmd.io/c/tutorials/

或是中文版 https://hackmd.io/c/tutorials-tw/

### What's the problem

Using inline MathJax/LaTeX formatting, the dollar symbol is not escaped. In multiline formatting, the dollar symbol is escaped as expected.

#### Current behaviour

Provide steps to reproduce:

1. Create a new document
2. Add inline MathJax formatting delimiters (`$...$`)
3. Add an escaped dollar symbol (`$\$$`)
4. See that it is not rendered correctly / is not escaped, and that the symbol is correctly rendered in multiline formatting

See note link: https://hackmd.io/@benschza/SykGbt_Vd

#### Expected behaviour

Symbol escaped correctly in both inline and multiline formatting.

#### Environment

I use HackMD on:

**Desktop**
- OS: macOS
- Browser: Firefox
- Browser Version: 86.0

#### Additional context

Add any other context about the problem here.