[HOTFIX] Fix the error that cannot find rag in tutorial

[DavdGao]

Description

Add rag section to tutorial.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has passed all tests
• Docstrings have been added/updated in Google Style
• Documentation has been updated
• Code is ready for review

[DavdGao]

@ZiTao-Li @FredericW Please note that to add a new section in tutorial, you must add it into the index.rst file, and you can pre-test it by following the guidance in https://github.com/modelscope/agentscope/blob/main/docs/README.md