Prompt fix for empty intermediate steps in summarization (#660)

Adding quotation marks around {text} avoids generating empty or
completely random responses from OpenAI davinci-003. Empty or completely
unrelated intermediate responses in summarization messes up the final
result or makes it very inaccurate.
The error from OpenAI would be: "The model predicted a completion that
begins with a stop sequence, resulting in no output. Consider adjusting
your prompt or stop sequences."
This fix corrects the prompting for summarization chain. This works on
API too, the images are for demonstrative purposes.
This approach can be applied to other similar prompts too. 

Examples:

1) Without quotation marks
![Screenshot from 2023-01-20
07-18-19](https://user-images.githubusercontent.com/22897470/213624365-9dfc18f9-5f3f-45d2-abe1-56de67397e22.png)

2) With quotation marks
![Screenshot from 2023-01-20
07-18-35](https://user-images.githubusercontent.com/22897470/213624478-c958e742-a4a7-46fe-a163-eca6326d9dae.png)

Author: vertinski

langchain/chains/summarize/map_reduce_prompt.py

@@ -4,7 +4,7 @@
 prompt_template = """Write a concise summary of the following:
 
 
-{text}
+"{text}"
 
 
 CONCISE SUMMARY:"""

langchain/chains/summarize/refine_prompts.py

@@ -21,7 +21,7 @@
 prompt_template = """Write a concise summary of the following:
 
 
-{text}
+"{text}"
 
 
 CONCISE SUMMARY:"""

langchain/chains/summarize/stuff_prompt.py

@@ -4,7 +4,7 @@
 prompt_template = """Write a concise summary of the following:
 
 
-{text}
+"{text}"
 
 
 CONCISE SUMMARY:"""