Update Readme

Author: Guantong

README.md

@@ -30,7 +30,7 @@ ___
 Add the following to your Cargo.toml file:
 
 ```toml
-openai_api_rust = "0.1.5"
+openai_api_rust = "0.1.8"
 ```
 
 Export your API key into the environment variables