fix typo in Iterable.expand docs

sdk/lib/core/iterable.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.
 
 part of dart.core;
 
 /**
  * A collection of values, or "elements", that can be accessed sequentially.
  *
  * The elements of the iterable are accessed by getting an [Iterator]
@@ skipping 156 matching lines @@
    *
    * This method returns a view of the mapped elements. As long as the
    * returned [Iterable] is not iterated over, the supplied function [test] will
    * not be invoked. Iterating will not cache results, and thus iterating
    * multiple times over the returned [Iterable] will invoke the supplied
    * function [test] multiple times on the same element.
    */
   Iterable<E> where(bool f(E element)) => new WhereIterable<E>(this, f);
 
   /**
-   * Expands each element of this [Iterable]into zero or more elements.
+   * Expands each element of this [Iterable] into zero or more elements.
    *
    * The resulting Iterable runs through the elements returned
    * by [f] for each element of this, in iteration order.
    *
    * The returned [Iterable] is lazy, and calls [f] for each element
    * of this every time it's iterated.
    */
   Iterable expand(Iterable f(E element)) =>
       new ExpandIterable<E, dynamic>(this, f);
 
@@ skipping 477 matching lines @@
  */
 abstract class BidirectionalIterator<E> implements Iterator<E> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }