fix: update qa_chain doc for "chai_type" (#5877)

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`load_qa_with_sources_chain` method already support four type of chain,
including `map_rerank`. update document to prevent any misunderstandings
😀.

![image](https://github.com/hwchase17/langchain/assets/6478745/325260b2-6121-4900-aef9-001febff811a)

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Fixes # (issue)
No, just update document.

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langchain/chains/qa_with_sources/loading.py

@@ -149,7 +149,7 @@ def load_qa_with_sources_chain(
     Args:
         llm: Language Model to use in the chain.
         chain_type: Type of document combining chain to use. Should be one of "stuff",
-            "map_reduce", and "refine".
+            "map_reduce", "refine" and "map_rerank".
         verbose: Whether chains should be run in verbose mode or not. Note that this
             applies to all chains that make up the final chain.