fix to avoid using property in add_entry

Author: nkanazawa1989

qiskit_experiments/database_service/utils.py

@@ -390,7 +390,7 @@ def add_entry(
         Raises:
             ValueError: When index is not unique in this table.
         """
-        with self.lock:
+        with self._lock:
             if index in self._container.index:
                 raise ValueError(f"Table index {index} already exists in the table.")