Fix unstable book

src/doc/unstable-book/src/language-features/macro-at-most-once-rep.md

@@ -6,7 +6,10 @@ With this feature gate enabled, one can use `?` as a Kleene operator meaning "0
 or 1 repetitions" in a macro definition. Previously only `+` and `*` were allowed.
 
 For example:
+
 ```rust
+#![feature(macro_at_most_once_rep)]
+
 macro_rules! foo {
     (something $(,)?) // `?` indicates `,` is "optional"...
         => {}