Fix readme

Author: Dongri Jin

README.md

@@ -18,9 +18,9 @@ The library needs to be configured with your account's secret key, which is avai
 $ export OPENAI_API_KEY=sk-xxxxxxx
 ```
 
-### Set OPENAI_API_ENDPOINT to environment variable (optional)
+### Set OPENAI_API_BASE to environment variable (optional)
 ```bash
-$ export OPENAI_API_ENDPOINT=https://api.openai.com/v1
+$ export OPENAI_API_BASE=https://api.openai.com/v1
 ```
 
 ### Create client