docs(eino): fix the bugs in the example code.

Add the missing required parameter Role: schema.User to the example code

Author: ryougi-misaki

content/zh/docs/eino/ecosystem_integration/chat_model/chat_model_openai.md

@@ -289,6 +289,7 @@ func main() {
         }
 
         multiModalMsg := &schema.Message{
+                Role: schema.User,
                 UserInputMultiContent: []schema.MessageInputPart{
                         {
                                 Type: schema.ChatMessagePartTypeText,