Fix: unify opendal config key from ‎schema to ‎scheme (#8232)

### 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

conf/service_conf.yaml

@@ -29,7 +29,6 @@ redis:
   db: 1
   password: 'infini_rag_flow'
   host: 'localhost:6379'
-
 # postgres:
 #   name: 'rag_flow'
 #   user: 'rag_flow'
@@ -61,7 +60,7 @@ redis:
 #   container_name: 'container_name'
 # The OSS object storage uses the MySQL configuration above by default. If you need to switch to another object storage service, please uncomment and configure the following parameters.
 # opendal:
-#   schema: 'mysql'  # Storage type, such as s3, oss, azure, etc.
+#   scheme: 'mysql'  # Storage type, such as s3, oss, azure, etc.
 #   config:
 #     oss_table: 'your_table_name'
 # user_default_llm: