Fix unknown attribute 'parent_category' for Category in demo generator

```ruby
❯ bin/rails demo_data:reset
user reset
bin/rails aborted!
ActiveModel::UnknownAttributeError: unknown attribute 'parent_category' for Category. (ActiveModel::UnknownAttributeError)

          raise UnknownAttributeError.new(self, k.to_s)
```

Follows: #1561

app/models/demo/generator.rb

@@ -92,9 +92,9 @@ def create_categories!
       end
 
       food = family.categories.find_by(name: "Food & Drink")
-      family.categories.create!(name: "Restaurants", parent_category: food)
-      family.categories.create!(name: "Groceries", parent_category: food)
-      family.categories.create!(name: "Alcohol & Bars", parent_category: food)
+      family.categories.create!(name: "Restaurants", parent: food)
+      family.categories.create!(name: "Groceries", parent: food)
+      family.categories.create!(name: "Alcohol & Bars", parent: food)
     end
 
     def create_merchants!