fix settings global docStoreConn for opensearch (#8885)

### What problem does this PR solve?
fix opensearch OSConnection init.
```
        docStoreConn = rag.utils.opensearch_conn.OSConnection()
```

### Type of change

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

Signed-off-by: zhanluxianshen <zhanluxianshen@163.com>

api/settings.py

     elif lower_case_doc_engine == "infinity":
         docStoreConn = rag.utils.infinity_conn.InfinityConnection()
     elif lower_case_doc_engine == "opensearch":
-        docStoreConn = rag.utils.opensearch_coon.OSConnection()
+        docStoreConn = rag.utils.opensearch_conn.OSConnection()
     else:
         raise Exception(f"Not supported doc engine: {DOC_ENGINE}")