### What problem does this PR solve?

This PR resolves the inconsistency in the opendal configuration where
both ‎`schema` and ‎`scheme` were used as keys. The code and
configuration file now consistently use ‎`scheme`, which helps prevent
configuration errors and runtime issues. This change improves code
clarity and maintainability.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

### Additional context
- Updated both ‎`conf/service_conf.yaml` and
‎`rag/utils/opendal_conn.py` to use ‎`scheme` instead of ‎`schema`
- No breaking changes to other configuration fields