[DOC] Update usage of AMS

`include` option is renamed as `embed_in_root`
in current stable version 0.9.
* https://github.com/rails-api/active_model_serializers/blob/0-9-stable/lib/active_model/serializer/association.rb#L11
Merge pull request #2383 from tricknotes/update-usage-of-ams

[DOC] Update usage of AMS