fix: error on example code

Author: sangheestyle

README.md

@@ -59,10 +59,10 @@ __Hello World__
 ```rust
 #![feature(async_await)]
 
-fn main() -> Result<(), failure::Error> {
+fn main() -> Result<(), std::io::Error> {
     let mut app = tide::App::new(());
     app.at("/").get(async move |_| "Hello, world!");
-    app.serve("127.0.0.1:8000")?;
+    Ok(app.serve("127.0.0.1:8000")?)
 }
 ```