Fix NavigationStackController docs (#3540)

Sources/ComposableArchitecture/Documentation.docc/Articles/MigrationGuides/MigratingTo1.13.md

@@ -107,7 +107,7 @@ class AppController: NavigationStackController {
       }
     }
 
-    self.model = model
+    self.store = store
   }
 }
 ```

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

@@ -683,7 +683,7 @@ class AppController: NavigationStackController {
       }
     }
 
-    self.model = model
+    self.store = store
   }
 }
 ```