Fix syntax in two doctests and un-ignored one
//! // Not going to compile because we're trying to borrow `s`,
//! // which lives *inside* the scope! :(
//! s.spawn(|_| println!("nested thread"));
-//! }});
+//! });
//! });
//! ```
//!
//! Fortunately, there is a solution. Every scoped thread is passed a reference to its scope as an
//! argument, which can be used for spawning nested threads:
//!
-//! ```ignore
+//! ```
//! use crossbeam_utils::thread;
//!
//! thread::scope(|s| {
//! s.spawn(|s| {
//! // Yay, this works because we're using a fresh argument `s`! :)
//! s.spawn(|_| println!("nested thread"));
-//! }});
+//! });
//! });
//! ```
//!
