refactor code (#583)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

api/apps/file_app.py

@@ -33,7 +33,7 @@ from api.settings import RetCode
 from api.utils.api_utils import get_json_result
 from api.utils.file_utils import filename_type
 from rag.nlp import search
-from rag.utils import ELASTICSEARCH
+from rag.utils.es_conn import ELASTICSEARCH
 from rag.utils.minio_conn import MINIO