Fix typo in Future.then documentation

lib/core/future.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
@@ skipping 52 matching lines @@
   bool get hasValue;
 
   /**
    * When this future is complete (either with a value or with an exception),
    * then [complete] is called with the future.
    * If [complete] throws an exception, it is ignored.
    */
   void onComplete(void complete(Future<T> future));
 
   /**
-   * If this future is complete and has a value, then [onValue] is called
+   * If this future is complete and has a value, then [onSuccess] is called
    * with the value.
    */
   void then(void onSuccess(T value));
 
   /**
    * If this future is complete and has an exception, then call [onException].
    *
    * If [onException] returns true, then the exception is considered handled.
    *
    * If [onException] does not return true (or [handleException] was never
@@ skipping 147 matching lines @@
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
 }