Merge pull request #181 from sangheestyle/fix_example_readme

docs: Correct sample code in README

Previously, the README example used `failure::Error`. While `failure` is a useful library in many scenarios, `app.serve()` returns `std::io::Result`. Removing `failure` allows the example to compile without additional changes, which could trip up new framework users.

Author: Nicholas

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")?)
 }
 ```