Update prompts.py

Fix a stipulation in the query_writer_instruction response format.

Author: cscandore

backend/src/agent/prompts.py

@@ -17,7 +17,7 @@ def get_current_date():
 - Query should ensure that the most current information is gathered. The current date is {current_date}.
 
 Format: 
-- Format your response as a JSON object with ALL three of these exact keys:
+- Format your response as a JSON object with ALL two of these exact keys:
    - "rationale": Brief explanation of why these queries are relevant
    - "query": A list of search queries