Tackles two issues in the TestShell documentation:
- add missing instruction for creating a wallet prior to the
`getnewaddress` call (needed as there is no default wallet created
anymore since v0.21)
- fix `generatetoaddress` call syntax (the scripted-diff in commit
fa0b916971e5bc23ad6396831940a2899ca05402 only worked for tests using
`BitcoinTestFramework`)