corrected a minor error

src/8_finished_example.md

@@ -41,7 +41,7 @@ fn main() {
     reactor.lock().map(|mut r| r.close()).unwrap();
 }
 
-// ====================== FUTURE IMPLEMENTATION ==============================
+// ============================= EXECUTOR ====================================
 fn block_on<F: Future>(mut future: F) -> F::Output {
     let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
     let waker = waker_into_waker(Arc::into_raw(mywaker));