fix headings

[Rich-Harris]

#1587 introduced a bad regex — we need the ? to make the pattern non-greedy. Without it, we're missing headings — https://svelte.dev/docs/kit/load currently looks like this...

...when it should look like this:

I also changed the hard-coded three backticks to a minimum of three, so that we can potentially do this sort of thing in future:

````md
this is a code fence:

```js
console.log('woooo!');
```
````

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Project	Deployment	Preview	Updated (UTC)
svelte-dev	Ready	Preview	Oct 4, 2025 11:26pm