Update website/src/pages/en/subgraphs/guides/x402-payments.mdx

Co-authored-by: Benoît Rouleau <1059139+benface@users.noreply.github.com>

Author: brandonleekramer

website/src/pages/en/subgraphs/guides/x402-payments.mdx

@@ -141,7 +141,7 @@ import { execute, GetPairsDocument } from './.graphclient'
 const result = await execute(GetPairsDocument, { first: 5 })
 ```
 
-**Environment Variables:**
+### Environment Variables
 
 - `X402_PRIVATE_KEY`: Wallet private key for payment signing
 - `X402_CHAIN`: `base` (mainnet) or `base-sepolia` (testnet)