Update recipe_book.dart

Author: vicb

Chapter_05/lib/recipe_book.dart

@@ -26,7 +26,7 @@ class RecipeBookController {
   List<Recipe> recipes = [];
 
   // Filter box
-  Map<String, bool> categoryFilterMap = {};
+  final categoryFilterMap = <String, bool>{};
   String nameFilterString = "";
 
   RecipeBookController(this._http) {

Chapter_06/lib/recipe_book.dart

@@ -34,7 +34,7 @@ class RecipeBookController {
   List<Recipe> get allRecipes => _allRecipes;
 
   // Filter box
-  Map<String, bool> categoryFilterMap = {};
+  final categoryFilterMap = <String, bool>{};
   String nameFilter = "";
 
   RecipeBookController(this._http, this.queryService) {