Update docs/build/apps/x402.mdx

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

Author: janewang

docs/build/apps/x402.mdx

@@ -40,7 +40,7 @@ You can use a facilitator to verify and settle x402 payments. Two options are av
 
 ### Coinbase x402 Facilitator
 
-Coinbase’s x402 facilitator supports Stellar on **testnet** with sponsored fees. Check which networks and options are supported:
+Coinbase’s x402 facilitator supports Stellar on **Testnet** with sponsored fees. Check which networks and options are supported:
 
 - **[x402 Facilitator Supported Networks](https://www.x402.org/facilitator/supported)** — Lists supported networks (including `stellar:testnet`) and facilitator configuration.