fix:fix open ai original validation. modify Tool's Function to pointer (#664)

Co-authored-by: caopengfei1 <caopengfei1@tal.com>

examples/completion-with-tool/main.go

@@ -35,7 +35,7 @@ func main() {
 	}
 	t := openai.Tool{
 		Type:     openai.ToolTypeFunction,
-		Function: f,
+		Function: &f,
 	}
 
 	// simulate user asking a question that requires the function