fix: repl example '<svelte:document>' (sveltejs#871)

Fixing the repl example '<svelte:document>'

Svelte Repl isn't working without `.toString()` on the selection.

apps/svelte.dev/content/examples/17-special-elements/04-svelte-document/+assets/App.svelte

@@ -1,7 +1,7 @@
 <script>
 	let selection = $state('');
 
-	const handleSelectionChange = (e) => (selection = document.getSelection());
+	const handleSelectionChange = (e) => (selection = document.getSelection().toString());
 </script>
 
 <svelte:document onselectionchange={handleSelectionChange} />