chore: remove API and private keys from .env file (#23)

Few things:

1. I don't think we want to expose any etherscan API keys we use. We also
   don't want to expose any private keys.
2. Not sure where this private key comes from, likely a ganache/anvil
   generated account, but nonetheless, I think the `.env.example` file
   is meant to be copied to `.env` and then changes are done there.

.env.example

@@ -1,3 +1,3 @@
-ETHERSCAN_API_KEY=ABC123ABC123ABC123ABC123ABC123ABC1
+ETHERSCAN_API_KEY=<YOUR_ETHERSCAN_KEY>
 SEPOLIA_URL=https://eth-sepolia.alchemyapi.io/v2/<YOUR ALCHEMY KEY>
-PRIVATE_KEY=0xabc123abc123abc123abc123abc123abc123abc123abc123abc123abc123abc1
+PRIVATE_KEY=<YOUR_PRIVATE_KEY>