Fix typo in Completer.sync documentation.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 7f4050cf8cb6dc661af812fb51bb6f0267b9de57..5d6a65009a4afae64ff5d150d7d8816bc876526f 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -625,7 +625,7 @@ abstract class Completer<T> {
    * behavior, but using a synchronous completer incorrectly can cause
    * otherwise correct programs to break.
    *
-   * An asynchronous completer is only intended for optimizing event
+   * A synchronous completer is only intended for optimizing event
    * propagation when one asynchronous event immediately triggers another.
    * It should not be used unless the calls to [complete] and [completeError]
    * are guaranteed to occur in places where it won't break `Future` invariants.