Fix bug in code sample

pkg/serialization/lib/serialization.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.
 
 /**
  * This provides a general-purpose serialization facility for Dart objects. A
  * [Serialization] is defined in terms of [SerializationRule]s and supports
  * reading and writing to different formats.
  *
  * Setup
  * =====
  * A simple example of usage is
  *
  *      var address = new Address();
  *      address.street = 'N 34th';
  *      address.city = 'Seattle';
  *      var serialization = new Serialization()
  *          ..addRuleFor(address);
- *      String output = serialization.write(address);
+ *      Map output = serialization.write(address);
  *
  * This creates a new serialization and adds a rule for address objects. Right
  * now it has to be passed an address instance because of limitations using
- * Address as a literal. Then we ask the Serialization to write the address
+ * Address as a literal. Then we ask the [Serialization] to write the address
  * and we get back a Map which is a [json]able representation of the state of
- * the address and related objects.
+ * the address and related objects. Note that while the output in this case
+ * is a [Map], the type will vary depending on which output format we've told
+ * the [Serialization] to use.
  *
  * The version above used reflection to automatically identify the public
  * fields of the address object. We can also specify those fields explicitly.
  *
  *      var serialization = new Serialization()
  *        ..addRuleFor(address,
  *            constructor: "create",
  *            constructorFields: ["number", "street"],
  *            fields: ["city"]);
  *
@@ skipping 431 matching lines @@
 }
 
 /**
  * An exception class for errors during serialization.
  */
 class SerializationException implements Exception {
   final String message;
   const SerializationException([this.message]);
   toString() => "SerializationException($message)";
 }