adds feature gate to doc-test (example)

Author: oliver-giersch

src/libstd/thread/mod.rs

@@ -427,6 +427,7 @@ impl Builder {
     /// # Examples
     ///
     /// ```
+    /// #![feature(thread_spawn_unchecked)]
     /// use std::thread;
     ///
     /// let builder = thread::Builder::new();