doc: fix readme azure example code (#571)

Author: whysage

README.md

@@ -90,7 +90,7 @@ openai.api_version = "2023-05-15"
 chat_completion = openai.ChatCompletion.create(deployment_id="deployment-name", model="gpt-3.5-turbo", messages=[{"role": "user", "content": "Hello world"}])
 
 # print the completion
-print(completion.choices[0].message.content)
+print(chat_completion.choices[0].message.content)
 ```
 
 Please note that for the moment, the Microsoft Azure endpoints can only be used for completion, embedding, and fine-tuning operations.