Fix typo in Etherscan API key env field

Author: lukaw3d

packages/nextjs/.env.example

@@ -10,7 +10,7 @@
 # You'll need to prefix the variables names with NEXT_PUBLIC_ if you want to access them on the client side.
 # More info: https://nextjs.org/docs/pages/building-your-application/configuring/environment-variables
 
-NEXT_PUBLIC_MAINNET_ETHERSCAN_API_KEY=
+NEXT_PUBLIC_ETHERSCAN_API_KEY=
 NEXT_PUBLIC_OPTIMISM_ETHERSCAN_API_KEY=
 NEXT_PUBLIC_POLYGON_ETHERSCAN_API_KEY=
 NEXT_PUBLIC_ARBITRUM_ETHERSCAN_API_KEY=