fix StiAutoloading example code

without the `.unscoped` call this can break if a base model happens to have a default scope.

guides/source/autoloading_and_reloading_constants.md

@@ -230,6 +230,7 @@ module StiPreload
       def preload_sti
         types_in_db = \
           base_class.
+            unscoped.
             select(inheritance_column).
             distinct.
             pluck(inheritance_column).