Update testnet addresses from Goerli to Sepolia (#35)

Updates Goerli references to Sepolia, now that CCTP has migrated.

Manually tested that the example script still works:
- depositForBurn:
https://sepolia.etherscan.io/tx/0x5ed89c2b0938d24f401d5c290a63b66a60cee27f625b054b667d84cd43c7c4b6
- receiveMessage:
https://testnet.snowtrace.io/tx/0x8b2144fa5c4292f7e9c27f2dde8f8b8fb38d693b27a22641ef37ca2ce98554cc?chainId=43113

---------

Co-authored-by: kevin-pv01 <134313281+kevin-pv01@users.noreply.github.com>

Author: walkerq

docs/README.md

@@ -28,7 +28,7 @@ The script has 5 steps:
     const approveTx = await usdcEthContract.methods.approve(ETH_TOKEN_MESSENGER_CONTRACT_ADDRESS, amount).send({gas: approveTxGas})
     ```
 
-2. Second step executes `depositForBurn` function on `TokenMessengerContract` deployed in [Goerli testnet](https://goerli.etherscan.io/address/0xd0c3da58f55358142b8d3e06c1c30c5c6114efe8)
+2. Second step executes `depositForBurn` function on `TokenMessengerContract` deployed in [Sepolia testnet](https://sepolia.etherscan.io/address/0x9f3B8679c73C2Fef8b59B4f3444d4e156fb70AA5)
     ```js
     const burnTx = await ethTokenMessengerContract.methods.depositForBurn(amount, AVAX_DESTINATION_DOMAIN, destinationAddressInBytes32, USDC_ETH_CONTRACT_ADDRESS).send();
     ``` 
@@ -52,7 +52,7 @@ The script has 5 steps:
     }
     ```
 
-5. Last step calls `receiveMessage` function on `TokenMessengerContract` deployed in [Avalanche Fuji Network](https://testnet.snowtrace.io/address/0xa9fb1b3009dcb79e2fe346c16a604b8fa8ae0a79) to receive USDC at AVAX address.
+5. Last step calls `receiveMessage` function on `MessageTransmitterContract` deployed in [Avalanche Fuji Network](https://testnet.snowtrace.io/address/0xa9fb1b3009dcb79e2fe346c16a604b8fa8ae0a79) to receive USDC at AVAX address.
 
     *Note: The attestation service is rate-limited, please limit your requests to less than 1 per second.*
     ```js

docs/index.js

@@ -27,9 +27,9 @@ const main = async() => {
     web3.eth.accounts.wallet.add(avaxSigner);
 
     // Testnet Contract Addresses
-    const ETH_TOKEN_MESSENGER_CONTRACT_ADDRESS = "0xd0c3da58f55358142b8d3e06c1c30c5c6114efe8";
-    const USDC_ETH_CONTRACT_ADDRESS = "0x07865c6e87b9f70255377e024ace6630c1eaa37f";
-    const ETH_MESSAGE_CONTRACT_ADDRESS = "0x1a9695e9dbdb443f4b20e3e4ce87c8d963fda34f"
+    const ETH_TOKEN_MESSENGER_CONTRACT_ADDRESS = '0x9f3B8679c73C2Fef8b59B4f3444d4e156fb70AA5';
+    const USDC_ETH_CONTRACT_ADDRESS = '0x1c7D4B196Cb0C7B01d743Fbc6116a902379C7238';
+    const ETH_MESSAGE_CONTRACT_ADDRESS = '0x80537e4e8bab73d21096baa3a8c813b45ca0b7c9';
     const AVAX_MESSAGE_TRANSMITTER_CONTRACT_ADDRESS = '0xa9fb1b3009dcb79e2fe346c16a604b8fa8ae0a79';
 
     // initialize contracts using address and ABI