Update the x402 demo site to official url

Author: janewang

docs/build/apps/x402.mdx

@@ -71,7 +71,7 @@ This version supports x402 v2 specification. For x402 v1 support, please use a p
 
 ## Learn more
 
-- [x402 Demo](https://x402-stellar-491bf9f7e30b.herokuapp.com/) - A demo site to a x402 transaction on Testnet
+- [x402 Demo](https://stellar.org/x402-demo) - A demo site to a x402 transaction on Stellar
 - [x402 protocol (Coinbase Developer Platform)](https://docs.cdp.coinbase.com/x402) — Official x402 protocol overview and spec
 - [x402 protocol specification](https://www.x402.org/) — x402 Specification and Whitepaper
 - [Coinbase x402 GitHub](https://github.com/coinbase/x402) — Official x402 Protocol GitHub Repo