Update the x402 demo site to official url (#2301)

* Update the x402 demo site to official url

* Update docs/build/apps/x402.mdx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update sidebar title

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: janewang

docs/build/apps/x402.mdx

@@ -1,7 +1,7 @@
 ---
 title: x402 on Stellar
 sidebar_position: 74
-sidebar_label: x402
+sidebar_label: x402 on Stellar
 description: "Use the x402 protocol for per-request HTTP payments on Stellar, with support for AI agents and APIs."
 ---
 
@@ -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 for an 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