fix variable name $PRIVATE_KEY

before this fix, the script revered to the private key in variable named $PRIVATE_KEY.

This made the snippet to not work is following the whole tutorial at https://docs.optimism.io/builders/app-developers/tutorials/cross-dom-bridge-erc20 where private key is declared under a different name

This change updates the variable name so the tutorial can work end to end by copy-pasting code snippets

fixes #1163

public/tutorials/standard-bridge-standard-token.sh

@@ -3,4 +3,4 @@ export TUTORIAL_RPC_URL=https://sepolia.optimism.io
 # Replace this with your L1 ERC-20 token if not using the testing token!
 export TUTORIAL_L1_ERC20_ADDRESS=0x5589BB8228C07c4e15558875fAf2B859f678d129
 
-cast send 0x4200000000000000000000000000000000000012 "createOptimismMintableERC20(address,string,string)" $TUTORIAL_L1_ERC20_ADDRESS "My Standard Demo Token" "L2TKN" --private-key $PRIVATE_KEY --rpc-url $TUTORIAL_RPC_URL --json | jq -r '.logs[0].topics[2]' | cast parse-bytes32-address
+cast send 0x4200000000000000000000000000000000000012 "createOptimismMintableERC20(address,string,string)" $TUTORIAL_L1_ERC20_ADDRESS "My Standard Demo Token" "L2TKN" --private-key $TUTORIAL_PRIVATE_KEY --rpc-url $TUTORIAL_RPC_URL --json | jq -r '.logs[0].topics[2]' | cast parse-bytes32-address