refactor: improve countries find_resource method

- Update `find_resource` method to use `scoped_collection.friendly.find(params[:id])`

This refactoring improves the find_resource method to work with friendly URLs, making it more robust and user-friendly.

app/admin/countries.rb

@@ -1,4 +1,9 @@
 ActiveAdmin.register Country do
+  controller do
+    def find_resource
+      scoped_collection.friendly.find(params[:id])
+    end
+  end
   # See permitted parameters documentation:
   # https://github.com/activeadmin/activeadmin/blob/master/docs/2-resource-customization.md#setting-up-strong-parameters
   #