[Example] Fix interrupted animation of Letters in development (#5694)

## Summary

Currently, in development the animation in Letters gets interrupted near
its end and stays like this, which doesn't look right. This doesn't
happen in production builds however - it seems to be related to
`setInterval` repetition time parameter - making it longer makes the
behavior uniform in both development and production.

## Test plan

💌

app/src/examples/LettersExample.tsx

@@ -44,7 +44,7 @@ export default function LettersExample() {
   useEffect(() => {
     const id = setInterval(() => {
       setState((s) => !s);
-    }, 2000);
+    }, 3500);
     return () => clearInterval(id);
   }, []);