fix an asp-action value (#98)

There is no "Update" in BasketController and it should be "Index".

src/Web/Views/Basket/Index.cshtml

@@ -68,7 +68,7 @@
                 }
                 <section class="esh-basket-item col-xs-push-8 col-xs-4">
                     <button class="btn esh-basket-checkout" name="updatebutton" value="" type="submit"
-                            asp-action="Update">[ Update ]</button>
+                            asp-action="Index">[ Update ]</button>
                     <input type="submit" asp-action="Checkout"
                            class="btn esh-basket-checkout"
                            value="[ Checkout ]" name="action" />