docs: fix $bindable hash link

Author: ignatiusmb

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -498,7 +498,7 @@ By default props are treated as readonly, meaning reassignments will not propaga
 
 Note that you can still use `export const` and `export function` to expose things to users of your component (if they're using `bind:this`, for example).
 
-### `$bindable()`
+## `$bindable()`
 
 To declare props as bindable, use `$bindable()`. Besides using them as regular props, the parent can (_can_, not _must_) then also `bind:` to them.