futures: fix typo in comment

corelib/src/future.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] obtain the value by passing a callback to [then]. For example:
@@ skipping 112 matching lines @@
   Completer();
 
   /** The future that will contain the value produced by this completer. */
   Future get future();
 
   /** Supply a value for [future]. */
   void complete(T value);
 
   /**
    * Indicate in [future] that an exception occured while trying to produce its
-   * value. The argument [exception] argument should not be [:null:].
+   * value. The argument [exception] should not be [:null:].
    */
   void completeException(Object exception);
 }
 
 /** Thrown when reading a future's properties before it is complete. */
 class FutureNotCompleteException implements Exception {
   FutureNotCompleteException() {}
   String toString() => "Exception: future has not been completed";
 }
 
@@ skipping 39 matching lines @@
 
     // Special case where all the futures are already completed,
     // trigger the value now.
     if (futures.length == 0) {
       completer.complete(values);
     }
 
     return completer.future;
   }
 }