Add missing default config value for :instance, instance_thumbnail

Follows up on b7fc61e17b

config/config.exs

@@ -186,6 +186,7 @@ config :pleroma, :instance,
   notify_email: "noreply@example.com",
   description: "Pleroma: An efficient and flexible fediverse server",
   background_image: "/images/city.jpg",
+  instance_thumbnail: "/instance/thumbnail.jpeg",
   limit: 5_000,
   chat_limit: 5_000,
   remote_limit: 100_000,