doc: test: update/fix TestShell example instructions
doc: test: update/fix TestShell example instructions

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`)

particl/particl-core,particl/particl-core,tecnovert/particl-core,particl/particl-core,particl/particl-core,bitcoin/bitcoin,fanquake/bitcoin,fanquake/bitcoin,bitcoin/bitcoin,bitcoin/bitcoin,bitcoin/bitcoin,tecnovert/particl-core,particl/particl-core,fanquake/bitcoin,bitcoin/bitcoin,fanquake/bitcoin,tecnovert/particl-core,tecnovert/particl-core,particl/particl-core,tecnovert/particl-core,fanquake/bitcoin,fanquake/bitcoin,tecnovert/particl-core,bitcoin/bitcoin