(issue #5163) Append reminder to nest multi-prompt router prompt output in JSON markdown code block, resolving JSON parsing error. (#8709)

Resolves occasional JSON parsing error when some predictions are passed
through a `MultiPromptChain`.

Makes [this
modification](https://github.com/langchain-ai/langchain/issues/5163#issuecomment-1652220401)
to `multi_prompt_prompt.py`, which is much cleaner than appending an
entire example object, which is another community-reported solution.

@hwchase17, @baskaryan

cc: @SimasJan

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

@@ -27,5 +27,5 @@ modifications are needed.
 << INPUT >>
 {{input}}
 
-<< OUTPUT >>
+<< OUTPUT (must include ```json at the start of the response) >>
 """