Sync svelte docs (#994)

sync svelte docs

Co-authored-by: Rich-Harris <1162160+Rich-Harris@users.noreply.github.com>

Author: github-actions[bot]

apps/svelte.dev/content/docs/svelte/98-reference/21-svelte-reactivity-window.md

@@ -47,7 +47,7 @@ on Firefox and Safari it won't.
 
 ```dts
 const devicePixelRatio: {
-	get current(): number;
+	get current(): number | undefined;
 };
 ```