Fix json parsing error of MULTI_PROMPT_ROUTER_TEMPLATE (#9944)

The output at times lacks the closing markdown code block. The prompt is
changed to explicitly request the closing backticks.

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libs/langchain/langchain/chains/router/multi_prompt_prompt.py

@@ -28,4 +28,5 @@ modifications are needed.
 {{input}}
 
 << OUTPUT (must include ```json at the start of the response) >>
+<< OUTPUT (must end with ```) >>
 """