Merge pull request #46 from mati865/master

Fix rust_2018_idioms in the snippet

Author: alexcrichton

src/editions/index.md

@@ -50,7 +50,7 @@ add this feature to your crate root:
 #![feature(rust_2018_preview)]
 
 // Opt in to warnings about new 2018 idioms
-#![feature(rust_2018_idioms)]
+#![warn(rust_2018_idioms)]
 ```
 
 and opt in to the new edition in your `Cargo.toml`: