Add missing comma in conv chat agent prompt json (#5573)

# Add missing comma in conversational chat agent prompt json

Inspired by: https://github.com/hwchase17/langchainjs/pull/1498

langchain/agents/conversational_chat/prompt.py

@@ -18,7 +18,7 @@ Markdown code snippet formatted in the following schema:
 
 ```json
 {{{{
-    "action": string \\ The action to take. Must be one of {tool_names}
+    "action": string, \\ The action to take. Must be one of {tool_names}
     "action_input": string \\ The input to the action
 }}}}
 ```