docs(readme): fix broken azure_ad notebook link (openai#781)

The notebook link in the main README pointed an old directory.

README.md

@@ -489,7 +489,7 @@ In addition to the options provided in the base `OpenAI` client, the following o
 - `azure_ad_token`
 - `azure_ad_token_provider`
 
-An example of using the client with Azure Active Directory can be found [here](https://github.com/openai/openai-python/blob/v1/examples/azure_ad.py).
+An example of using the client with Azure Active Directory can be found [here](https://github.com/openai/openai-python/blob/main/examples/azure_ad.py).
 
 ## Versioning