"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.

Commencez par reproduire le problème sur hackmd.io avec un aperçu des diapositives et le texte d’exemple contenant un point d’interrogation. Suivez le comportement du menu des raccourcis clavier et vérifiez qu’il se ferme lorsque vous passez à une autre diapositive ou que vous continuez à saisir du texte, sans nécessiter un second point d’interrogation.

Rédigé par le modèle d'indexation à partir du texte de l'issue.