Improve open graph images

Use the more generic og:image instead of twitter:image to make the preview image show up in more contexts

Signed-off-by: Wolfr <johan.ronsse@gmail.com>

site/src/template.html

@@ -14,7 +14,7 @@
 	<meta name='twitter:card' content='summary_large_image'>
 	<meta name='twitter:site' content='@sveltejs'>
 	<meta name='twitter:creator' content='@sveltejs'>
-	<meta name='twitter:image' content='https://svelte.dev/images/twitter-card.png'>
+	<meta name='og:image' content='https://svelte.dev/images/og-card.png'>
 
 	<!-- Sapper generates a <style> tag containing critical CSS
 	     for the current page. CSS for the rest of the app is