### What problem does this PR solve?

Since `import markdown.markdown` has been changed to `import markdown`
in `rag/app/naive.py`, previous code for converting markdown tables
would call a markdown module instead of a callable function. This cause
error.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):