Update react_agent.py

Slight modification to the format_instruction to improve the success rate of `RegexTaggedContentParser` parsing the model API output.

src/agentscope/agents/react_agent.py

@@ -109,7 +109,7 @@ def __init__(
 
         # Initialize a parser object to formulate the response from the model
         self.parser = RegexTaggedContentParser(
-            format_instruction="""Respond with specific tags as outlined below:
+            format_instruction="""Respond with specific tags as outlined below in json format:
 <thought>{what you thought}</thought>
 <function>{the function name you want to call}</function>
 <{argument name}>{argument value}</{argument name}>