Update StackBasedNavigation.md (pointfreeco#2499)

Fixed `self.path` to `state.path`

Author: x-0o0

Sources/ComposableArchitecture/Documentation.docc/Articles/StackBasedNavigation.md

@@ -213,7 +213,7 @@ additional logic, such as popping the "edit" feature and saving the edited item
 
 ```swift
 case let .path(.element(id: id, action: .editItem(.saveButtonTapped))):
-  guard case let .editItem(editItemState) = self.path[id: id]
+  guard case let .editItem(editItemState) = state.path[id: id]
   else { return .none }
 
   state.path.pop(from: id)