Fix an outdated method call in example code for oauth2.

pkg/oauth2/lib/oauth2.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 client library for authenticating with a remote service via OAuth2 on
 /// behalf of a user, and making authorized HTTP requests with the user's OAuth2
 /// credentials.
 ///
 /// OAuth2 allows a client (the program using this library) to access and
 /// manipulate a resource that's owned by a resource owner (the end user) and
@@ skipping 38 matching lines @@
 ///     // client. The redirection will include the authorization code in the
 ///     // query parameters.
 ///     final redirectUrl = Uri.parse(
 ///         "http://my-site.com/oauth2-redirect");
 ///     
 ///     var credentialsFile = new File("~/.myapp/credentials.json");
 ///     return credentialsFile.exists().then((exists) {
 ///       // If the OAuth2 credentials have already been saved from a previous
 ///       // run, we just want to reload them.
 ///       if (exists) {
-///         return credentialsFile.readAsText().then((json) {
+///         return credentialsFile.readAsString().then((json) {
 ///           var credentials = new oauth2.Credentials.fromJson(json);
 ///           return new oauth2.Client(identifier, secret, credentials);
 ///         });
 ///       }
 ///     
 ///       // If we don't have OAuth2 credentials yet, we need to get the
 ///       // resource owner to authorize us. We're assuming here that we're a
 ///       // command-line application.
 ///       var grant = new oauth2.AuthorizationCodeGrant(
 ///           identifier, secret, authorizationEndpoint, tokenEndpoint);
@@ skipping 30 matching lines @@
 ///         }).then((file) => file.close()).then((_) => result);
 ///       });
 ///     }).then(print);
 library oauth2;
 
 export 'src/authorization_code_grant.dart';
 export 'src/client.dart';
 export 'src/credentials.dart';
 export 'src/authorization_exception.dart';
 export 'src/expiration_exception.dart';