docs(advanced-alchemy): fix attribute name in services example (#589)

The example for Complex Operation in the Usage / Services section of the Advanced Alchemy docs use a `default_load_options` attribute. To my understanding, this should be `loader_options` instead.

Author: ywallis

docs/usage/services.rst

@@ -116,7 +116,7 @@ The code below shows a service coordinating posts and tags.
 
     class PostService(SQLAlchemyAsyncRepositoryService[Post, PostRepository]):
 
-        default_load_options = [Post.tags]
+        loader_options = [Post.tags]
         repository_type = PostRepository
         match_fields = ["name"]