Merge pull request rails#55098 from thehale/docs_extract_a_concern

Match code extracted into a concern in guides [skip ci]

Author: fatkodima

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