Fix rust_2018_idioms in the snippet

mati865 · mati865 · commit 5f4cd09aa8a7 · 2018-06-25T20:19:57.000+02:00
diff --git a/src/editions/index.md b/src/editions/index.md
@@ -43,7 +43,7 @@ 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`:
