fix: correct malformed code block marker in system prompt template

prompts.yaml

@@ -27,8 +27,7 @@ prompt_templates:
       {% endfor %}
 
       Here are the rules you should always follow to solve your task:
-      1. Always provide a 'Thought:' sequence, and a 'Code:
-      ```py' sequence ending with '```<end_code>' sequence, else you will fail.
+      1. Always provide a 'Thought:' sequence, and a 'Code:' sequence with ```py at the start and ```<end_code> at the end, else you will fail.
       2. Use only variables that you have defined!
       3. Always use the right arguments for the tools.
       4. Take care to not chain too many sequential tool calls in the same code block.