refine file select prompt

src/Infrastructure/BotSharp.Core/data/agents/6745151e-6d46-4a02-8de4-1c4f21c7da95/templates/util-file-select_file_instruction.liquid

@@ -4,7 +4,7 @@ Please take a look at the files in the [FILES] section from the conversation and
 ** If no files are selected, you must output an empty list [].
 ** You may need to look at the file_name as a reference to find the correct file id or ids.
 
-Here is the JSON format to use:
+*** Your output must be in the following JSON format:
 {
     "selected_ids": a list of id selected from the [FILES] section
 }