update DispatchFromDyn doctest

Author: mikeyhew

src/libcore/ops/unsize.rs

@@ -84,6 +84,9 @@ impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
 /// example impl:
 ///
 /// ```
+/// # #![feature(dispatch_from_dyn, unsize)]
+/// # use std::{ops::DispatchFromDyn, marker::Unsize};
+/// # struct Rc<T: ?Sized>(::std::rc::Rc<T>);
 /// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>
 /// where
 ///     T: Unsize<U>,