Fixed the invalid usage of primary_abstract_class

This is a method of [ActiveRecord::Inheritance::ClassMethods](https://api.rubyonrails.org/classes/ActiveRecord/Inheritance/ClassMethods.html#method-i-primary_abstract_class), but the guide treats it as an attribute.

guides/source/active_record_multiple_databases.md

@@ -131,7 +131,7 @@ should share a connection with.
 
 ```ruby
 class PrimaryApplicationRecord < ActiveRecord::Base
-  self.primary_abstract_class = true
+  primary_abstract_class
 end
 ```