Fix code example error in stateful docs

[nmbrgts]

Hello again! I noticed this small mistake in the stateful docs: the docs suggest settings can be set at FiniteStateMachine.settings when it is actually at FiniteStateMachine.TestCase.settings.

I realize that this is a pretty light PR. If that is a problem, I'd be happy to flesh this PR out with additional doc improvements. I could add some notes on using @settings(...) with state machines (... which I probably should have done in my last PR). I'm open to other suggestions too 😄

[Zac-HD]

Small PRs are great! If you had multiple one-line changes which each needed a release we would probably consolidate them, but for documentation or test changes the only limit is waiting for CI 😄

[Zac-HD]

FYI this also prompted me to ask "how could we help users get this right" (one of my favorite questions), and #1643 has my notes on how to enforce it at runtime. That's a whole separate problem to fixing the docs though!

[nmbrgts]

Thanks for the clarification on small PRs! I'll take a look at your notes as well.