Prefer .zero? over == 0 to match code extracted into a concern

The code written in the "In Stock Email Notifications" section uses `.zero?` to check if the inventory was empty. In contrast the code snippet shown in the following section, "Extracting a Concern", changes the comparison to `== 0`.

Consistency one way or the other would help reduce confusion.

Author: thehale

guides/source/getting_started.md

@@ -2315,7 +2315,7 @@ module Product::Notifications
   end
 
   def back_in_stock?
-    inventory_count_previously_was == 0 && inventory_count > 0
+    inventory_count_previously_was.zero? && inventory_count > 0
   end
 
   def notify_subscribers