Fix readme

Author: Dongri Jin

README.md

@@ -64,7 +64,6 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             role: chat_completion::MessageRole::user,
             content: String::from("What is Bitcoin?"),
             name: None,
-            function_call: None,
         }],
     );
     let result = client.chat_completion(req)?;