Fix doctest better

Author: eholk

library/core/src/iter/sources/generator.rs

@@ -9,6 +9,8 @@
 ///
 /// ```
 /// #![feature(iter_macro, coroutines)]
+/// # #[cfg(not(bootstrap))]
+/// # {
 ///
 /// let it = std::iter::iter!{|| {
 ///     yield 1;
@@ -17,6 +19,7 @@
 /// } }();
 /// let v: Vec<_> = it.collect();
 /// assert_eq!(v, [1, 2, 3]);
+/// # }
 /// ```
 #[unstable(feature = "iter_macro", issue = "none", reason = "generators are unstable")]
 #[allow_internal_unstable(coroutines, iter_from_coroutine)]