Fix typo in stuff_prompt's system_template (#12063)

- **Description:** 

Add missing apostrophe in `user's` in stuff_prompt's system_template.
The first sentence in the system template went from:

> Use the following pieces of context to answer the users question.

to

> Use the following pieces of context to answer the user's question.

- **Issue:** 
- **Dependencies:** none
- **Tag maintainer:** @baskaryan
- **Twitter handle:** ojohnnyo

libs/langchain/langchain/chains/question_answering/stuff_prompt.py

@@ -17,7 +17,7 @@ PROMPT = PromptTemplate(
     template=prompt_template, input_variables=["context", "question"]
 )
 
-system_template = """Use the following pieces of context to answer the users question. 
+system_template = """Use the following pieces of context to answer the user's question. 
 If you don't know the answer, just say that you don't know, don't try to make up an answer.
 ----------------
 {context}"""