put the code to anterior position

correct code exist in the wrong place, it will lead a compiling warning

source/chapter2/14_Initialization.md

@@ -837,9 +837,10 @@ if let bowTie = Product(name: "bow tie") {
 class CartItem: Product {
     let quantity: Int!
     init?(name: String, quantity: Int) {
+        self.quantity = quantity
         super.init(name: name)
         if quantity < 1 { return nil }
-        self.quantity = quantity
+        
     }
 }
 ```