Collapsible summaries involving headers are rendered on 2 lines instead of one

[nikelborm]

Describe the bug

The bug appears on this page: https://npmx.dev/package/fetch-github-folder

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The HTML, inlined in markdown, which produces this behaviour:

<details>
<summary>

### Install package with only functions from [JSR](https://jsr.io/@nikelborm/git-dl)

</summary>

Unfortunately JSR doesn't support publishing executables yet, so you can install
only script library with functions that will allow you to fetch github folder
from other scripts.

```bash
npx jsr add @nikelborm/git-dl
` ``

</details>

An example of expected behavior in JSR: https://jsr.io/@nikelborm/fetch-github-folder

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Or on github: https://github.com/nikelborm/effect-garden/tree/main/packages/git-dl

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