fixes misplaced semicolon

Author: oliver-giersch

src/libstd/thread/mod.rs

@@ -437,8 +437,8 @@ impl Builder {
     /// let handler = unsafe {
     ///     builder.spawn_unchecked(move || {
     ///         println!("x = {}", *thread_x);
-    ///     }).unwrap();
-    /// }
+    ///     }).unwrap()
+    /// };
     ///
     /// // caller has to ensure `join()` is called, otherwise
     /// // it is possible to access freed memory if `x` gets