Fix judge prompt: escape JSON braces

prompts/ragbench_judge_prompt.txt

@@ -97,22 +97,22 @@ outside the JSON. Do not wrap the JSON in backticks.
 
 Example (structure ONLY, values are illustrative):
 
-{
+{{
   "relevance_explanation": "....",
   "all_relevant_sentence_keys": ["D1_S1", "D1_S3"],
   "overall_supported_explanation": "....",
   "overall_supported": true,
   "sentence_support_information": [
-    {
+    {{
       "sentence_key": "D1_S1",
       "is_supported": true,
       "explanation": "The answer’s first claim comes from this sentence."
-    },
-    {
+    }},
+    {{
       "sentence_key": "D1_S3",
       "is_supported": false,
       "explanation": "The answer misrepresents this sentence."
-    }
+    }}
   ],
   "all_utilized_sentence_keys": ["D1_S1"]
-}
+}}