Describe a bug.
The copied HTML data from Editor.js have wrong `

`.
`

` can't contain `

`~`

` tags by HTML syntax rule.
https://stackoverflow.com/questions/15656785/should-a-heading-be-inside-or-outside-a-p

Steps to reproduce:
1. Instert the following debug code.

`src/components/modules/rectangleSelection.ts` line 307
```typescript
const textHTML = fakeClipboard.innerHTML;

console.log(textHTML); // debug code

e.clipboardData.setData('text/plain', textPlain);
```

2. Copy an heading block by `ctrl + c` in `example/example-dev.html`


3. The wrong HTML is output to console.

```html

Editor.js

Expected behavior: `