[TOC] does not work in presentation mode

### 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

#### Current behaviour
* [TOC] tag works perfectly in book mode, but in presentation mode, it does not display anything.

Provide steps to reproduce:

1. add [TOC] in md and separate it using \--- like
```
---

[TOC]

---

OTHER CONTENT
```
2. enter presentation mode

#### Expected behaviour

* display the correct table of content as it does in book mode.

#### Environment

I use HackMD on:

**Desktop**
- OS: Windows
- Browser: Chrome
- Browser Version: 88.0.4324.182 (Official Build) (64-bit)