Update load_qa_chain() docstring (#2900)

Seems to be missing `map_rerank` as a potential argument of
`chain_type`

langchain/chains/question_answering/__init__.py

@@ -196,7 +196,7 @@ def load_qa_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", "map_rerank", and "refine".
         verbose: Whether chains should be run in verbose mode or not. Note that this
             applies to all chains that make up the final chain.
         callback_manager: Callback manager to use for the chain.