Fix mypy index property access in opensearch example (#35691)

The new opensearch-py 2.4.1 added mypy stubs in
opensearch-project/opensearch-py#563 and
they detected an error in our example (and they cause mypy error
when the new version is used)

GitOrigin-RevId: dffe04ada4c05a3e29422f6ea92625c6e040bf10

tests/system/providers/opensearch/example_opensearch.py

@@ -110,7 +110,7 @@ def load_connections():
         query={"query": {"bool": {"must": {"match": {"message": "hello world"}}}}},
     )
     search = Search()
-    search.index = INDEX_NAME
+    search._index = [INDEX_NAME]
     search_object = search.filter("term", logger="airflow").query("match", message="hello airflow")
 
     search_high_level = OpenSearchQueryOperator(task_id="high_level_query", search_object=search_object)