🐛 Docs Fixes [2 one-liners, examples broken] (#8519)

## Description: 
   
1)Map reduce example in docs is missing an important import statement.
Figured other people would benefit from being able to copy 🍝 the code.

2)RefineDocumentsChain example also broken.

## Issue: 

None

## Dependencies:

None. One liner.

## Tag maintainer:

@baskaryan

## Twitter handle: 

I mean, it's a one line fix lol. But @will_thompson_k is my twitter
handle.

docs/snippets/modules/chains/popular/summarize.mdx

@@ -184,6 +184,7 @@ You can also use prompt with multi input. In this example, we will use a MapRedu
 ```python
 from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain
 from langchain.chains.combine_documents.stuff import StuffDocumentsChain
+from langchain.chains import ReduceDocumentsChain
 
 map_template_string = """Give the following python code information, generate a description that explains what the code does and also mention the time complexity.
 Code:

libs/langchain/langchain/chains/combine_documents/refine.py

@@ -53,7 +53,7 @@ class RefineDocumentsChain(BaseCombineDocumentsChain):
             prompt = PromptTemplate.from_template(
                 "Summarize this content: {context}"
             )
-            llm_chain = LLMChain(llm=llm, prompt=prompt)
+            initial_llm_chain = LLMChain(llm=llm, prompt=prompt)
             initial_response_name = "prev_response"
             # The prompt here should take as an input variable the
             # `document_variable_name` as well as `initial_response_name`
@@ -61,7 +61,7 @@ class RefineDocumentsChain(BaseCombineDocumentsChain):
                 "Here's your first summary: {prev_response}. "
                 "Now add to it based on the following context: {context}"
             )
-            llm_chain_refine = LLMChain(llm=llm, prompt=prompt_refine)
+            refine_llm_chain = LLMChain(llm=llm, prompt=prompt_refine)
             chain = RefineDocumentsChain(
                 initial_llm_chain=initial_llm_chain,
                 refine_llm_chain=refine_llm_chain,