[TOC maxLevel=2] is rendered as plain text instead of generating a table of contents

### 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/

or Chinese version: https://hackmd.io/c/tutorials-tw/

### What's the problem

#### Current behaviour

`[TOC]` works, but `[TOC maxLevel=2]` is rendered as plain text and does not generate a table of contents.

Reproducible content:

```md
[TOC maxLevel=2]

### Section
- ### Case1
- ### Case2
- ### Case3
````

Actual rendered result:

```md
[TOC maxLevel=2]
```

It is displayed as plain text instead of being parsed as TOC syntax.

Provide steps to reproduce:

1. Go to [https://hackmd.io](https://hackmd.io)

2. Create a new note

3. Paste the content below:

```md
[TOC maxLevel=2]

### Section
- ### Case1
- ### Case2
- ### Case3
```

4. Switch to preview / rendered view

5. Observe that `[TOC maxLevel=2]` is shown as plain text

> Here is an example: https://hackmd.io/AkRF_wKqQWqpo_MJ32Veww

#### Expected behaviour

`[TOC maxLevel=2]` should be recognized as valid TOC syntax and generate a table of contents, instead of being rendered as plain text.

#### Environment

I use HackMD on:

**Desktop**

* OS: Windows / Ubuntu 24.04 / Android 14
* Browser: Chrome 147.0.7727.56 , FireFox 143.0（64 位元）

**Mobile**

* Device: Xiaomi Pro 13
* OS: Android 14
* Browser: Chrome 146.0.7880.177

#### Additional context

The official tutorial documents `[TOC maxLevel=1]` as supported syntax, so this behavior appears inconsistent with the documentation.