Update metadata.json: Apply 2026-05-19 audit template fix

metadata.json

@@ -10,7 +10,7 @@
         "single_image_start"
       ],
       "task_type": "vqa",
-      "prompt_template": "<image>{{ question }}\n",
+      "prompt_template": "Please first solve the problem step by step, then put your final answer or a single letter (if it is a multiple choice question) in one \"\\boxed{}\". \n<image>{{ question }}",
       "mapping_from_source": {
         "media": {
           "from": "image",