site: fix twitter card (#11978)

Author: benmccann

sites/kit.svelte.dev/src/app.html

@@ -12,8 +12,8 @@
 		<meta name="twitter:card" content="summary" />
 		<meta name="twitter:site" content="@sveltejs" />
 		<meta name="twitter:creator" content="@sveltejs" />
-		<meta name="twitter:image" content="/images/twitter-thumbnail.jpg" />
-		<meta name="og:image" content="/images/twitter-thumbnail.jpg" />
+		<meta name="twitter:image" content="https://kit.svelte.dev/images/twitter-thumbnail.jpg" />
+		<meta name="og:image" content="https://kit.svelte.dev/images/twitter-thumbnail.jpg" />
 
 		<link
 			rel="search"