fix: make example code compile (useless, but why not)

source/test.md

@@ -29,12 +29,14 @@ mydata: 123
 text text text `inline code` text text text
 
 ```rs
+// cargo add tokio -F macros,rt-multi-thread,fs
+
 #[tokio::main]
 async fn main() -> std::io::Result<()> {
-  let cfg = tokio::fs::read_to_string("config.txt");
+    let cfg = tokio::fs::read_to_string("config.txt").await?;
-  let cfg: Vec<std::borrow::Cow<str>> = cfg.split(';').collect();
+    let cfg: Vec<&str> = cfg.split(';').collect();
-  println!(cfg.first().unwrap_or("hello".into()));
+    println!("{}", cfg.first().unwrap_or(&"hello"));
-  Ok(())
+    Ok(())
 }
 ```