message: fix non-compile-y doctest
diff: //! span is _completed_. Consider, for example, a future which has an associated
  //! span and enters that span every time it is polled:
  //! ```rust
  +//! # extern crate futures;
  +//! # use futures::{Future, Poll, Async};
  //! struct MyFuture {
  //! // data
  //! span: tokio_trace::Span,
  //! fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
  //! self.span.enter(|| {
  //! // Do actual future work
  +//! # Ok(Async::Ready(()))
  //! })
  //! }
  //! }
  