Fix an outdated method call in example code for oauth2.

pkg/oauth2/lib/oauth2.dart

Index: pkg/oauth2/lib/oauth2.dart
diff --git a/pkg/oauth2/lib/oauth2.dart b/pkg/oauth2/lib/oauth2.dart
index 4dfaadd21ce13db3d00ae1e91a428d4b9a76d6e1..7fdb205557a5e45879a76308e79e3d5f54f6f4ec 100644
--- a/pkg/oauth2/lib/oauth2.dart
+++ b/pkg/oauth2/lib/oauth2.dart
@@ -56,7 +56,7 @@
 ///       // 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);
 ///         });