Fix example that includes serialized ToolReturnPart

Author: DouweM

docs/tools.md

@@ -108,6 +108,7 @@ print(dice_result.all_messages())
                 tool_call_id='pyd_ai_tool_call_id',
                 timestamp=datetime.datetime(...),
                 part_kind='tool-return',
+                multi_modal_file_index=None,
             )
         ],
         instructions=None,
@@ -134,6 +135,7 @@ print(dice_result.all_messages())
                 tool_call_id='pyd_ai_tool_call_id',
                 timestamp=datetime.datetime(...),
                 part_kind='tool-return',
+                multi_modal_file_index=None,
             )
         ],
         instructions=None,