"Keyboard Shortcut" menu gets open (and doesn't close) when typing a question mark in the slide text.

### 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

During slide creation, shortcut menu gets opened, and doesn't get closed.

#### Current behaviour

Typing a slide text that contains a question mark the "Keyboard Shortcuts " menu permanently.

Provide steps to reproduce:

1. Start a hackmd document, with "slides" preview:
```
---
type: slide
---
```

2. Write a line with a question mark
```
Is this a slide?
```

3. Outcome: the "Keyboard Shortcuts" menu open, and hide the slide text.

#### Expected behaviour

The shortcut menu should disappear when moving to another slide, or typing more text

#### Environment

I use HackMD on:

**Desktop**
- OS: [macOS]
- Browser: [chrome]
- Browser Version: [71]

#### Workaround

Currently, the only way to close this dialog is to typing **another** question mark (and delete it) - but that is a bit counter-intuitive.