Document valid network values in .env.example (#2)

Valid networks are: base-sepolia (testnet), base (mainnet)
Note: "base-mainnet" is not supported.

Co-authored-by: Juntao Yuan <juntao@secondstate.io>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

.env.example

@@ -6,6 +6,7 @@ APP_PORT=8000
 APP_BASE_URL=http://localhost:8000
 
 # x402 Payment Settings
+# Valid networks: base-sepolia (testnet), base (mainnet)
 NETWORK=base-sepolia
 FACILITATOR_URL=https://x402f1.secondstate.io
 MAX_TIMEOUT_SECONDS=60