Fix set_total! method (#89)

rails6/en/chapter07-placing-orders.adoc

@@ -504,7 +504,7 @@ We can now add the implementation:
 class Order < ApplicationRecord
   # ...
   def set_total!
-    self.total = products.sum :price
+    self.total = self.products.map(&:price).sum
   end
 end
 ----

rails6/es/chapter07-placing-orders.adoc

@@ -503,7 +503,7 @@ Ahora podemos añadir la implementación:
 class Order < ApplicationRecord
   # ...
   def set_total!
-    self.total = products.sum :price
+    self.total = self.products.map(&:price).sum
   end
 end
 ----

rails6/fr/chapter07-placing-orders.adoc

@@ -489,7 +489,7 @@ Nous pouvons maintenant ajouter l'implémentation:
 class Order < ApplicationRecord
   # ...
   def set_total!
-    self.total = self.products.sum :price
+    self.total = self.products.map(&:price).sum
   end
 end
 ----