Update README.md sample code to 2018 edition

fitzgen · web-flow · commit 5233139b31e2 · 2019-05-21T19:22:34.000-07:00
diff --git a/README.md b/README.md
@@ -12,7 +12,6 @@
 Import JavaScript things into Rust and export Rust things to JavaScript.
 
 ```rust
-extern crate wasm_bindgen;
 use wasm_bindgen::prelude::*;
 
 // Import the `window.alert` function from the Web.
