ch07 notebook: code fix + typo

[ArnaudBrousseau]

This branch contains a few minor fixes to the chapter 7 code:

• a fix for the second cell: it needs to parse on testnet instead of mainnet (a 404/crash is triggered when trying to parse on mainnet)
• a code var rename (transaction => tx_obj) to fix cell execution (transaction is not declared anywhere)
• a simple typo fix (creat => create)