Update the Unstable Book to use #[rustc_auto_trait]

Author: fmease

src/doc/unstable-book/src/language-features/auto-traits.md

@@ -26,7 +26,8 @@ Example:
 #![feature(negative_impls)]
 #![feature(auto_traits)]
 
-auto trait Valid {}
+#[rustc_auto_trait]
+trait Valid {}
 
 struct True;
 struct False;
@@ -48,7 +49,7 @@ fn main() {
 
 ## Automatic trait implementations
 
-When a type is declared as an `auto trait`, we will automatically
+When a type is declared as an auto trait, we will automatically
 create impls for every struct/enum/union, unless an explicit impl is
 provided. These automatic impls contain a where clause for each field
 of the form `T: AutoTrait`, where `T` is the type of the field and