Change Counter to CounterFeature in StackBasedNavigation docs

[BruceMcRooster]

I believe this was just a small typo, but I may be mistaken.
The example code in StackBasedNavigation.md referred to a Counter type, which I think was supposed to be the CounterFeature type created in the previous code block.

---

The reducer CounterFeature was declared

@Reducer
struct CounterFeature {
  // ...
}

and the main Feature was supposed to contain it in a StackState, to show testing stack-based navigation.

@Reducer
struct Feature {
  @ObservableState
  struct State: Equatable {
    var path = StackState<Path.State>()
  }
  enum Action {
    case path(StackActionOf<Path>)
  }

  @Reducer  
  struct Path {
    enum State: Equatable { case counter(Counter.State) }
    enum Action { case counter(Counter.Action) }
    var body: some ReducerOf<Self> {
      Scope(state: \.counter, action: \.counter) { Counter() }
    }
  }

  // ...
}

However, the Counter type referenced was not declared anywhere else, and the future examples of testing construct the Path using a CounterFeature.

@Test
func dismissal() {
  let store = TestStore(
    initialState: Feature.State(
      path: StackState([
        CounterFeature.State(count: 3)
      ])
    )
  ) {
    CounterFeature()
  }
}