Small tweaks to readable store example

The current time should be stored by the callback when the first subscriber comes, and not when the readable is first created

site/content/docs/03-run-time.md

@@ -287,7 +287,8 @@ The second argument to `readable` is the same as the second argument to `writabl
 ```js
 import { readable } from 'svelte/store';
 
-const time = readable(new Date(), set => {
+const time = readable(null, set => {
+  set(new Date());
 	const interval = setInterval(() => {
 		set(new Date());
 	}, 1000);