fixing docstring (#2757)

Author: davidsbatista

integrations/opensearch/src/haystack_integrations/document_stores/opensearch/document_store.py

@@ -1351,17 +1351,21 @@ async def get_metadata_fields_info_async(self) -> dict[str, dict[str, str]]:
 
         If we populated the index with documents like:
 
+        ```python
             Document(content="Doc 1", meta={"category": "A", "status": "active", "priority": 1})
             Document(content="Doc 2", meta={"category": "B", "status": "inactive"})
+        ```
 
         This method would return:
 
+        ```python
             {
                 'content': {'type': 'text'},
                 'category': {'type': 'keyword'},
                 'status': {'type': 'keyword'},
                 'priority': {'type': 'long'},
             }
+        ```
 
         :returns: The information about the fields in the index.
         """