chore(tests/dummy/app/templates/demo/select): fixes topping selection.

Author: matthewhartstonge

tests/dummy/app/templates/demo/select.hbs

@@ -41,7 +41,7 @@
       {{size}}
     </PaperSelect>
     <PaperSelect @options={{this.groupedToppings}} @label="Topping" @selected={{this.topping}} @onChange={{action (mut this.topping)}} as |topping|>
-      {{this.topping}}
+      {{topping}}
     </PaperSelect>
     {{! END-SNIPPET }}
   </div>