如何在Rails中更改Gemfile时修复错误？

Question

我是rails的新手，当我编辑下面的Gemfile时，我一直遇到错误。它也可以在本教程的清单2.1中找到：http://ruby.railstutorial.org/chapters/a-demo-app#sec-planning_the_application

source 'https://rubygems.org'
gem 'rails', '3.2.7'
group :development do
  gem 'sqlite3', '1.3.5'
end
# Gems used only for assets and not required # in production environments by default. 
group :assets do
  gem 'sass-rails',   '3.2.4'
  gem 'coffee-rails', '3.2.2'
  gem 'uglifier', '1.2.3'
end
gem 'jquery-rails', '2.0.0'
group :production do
  gem 'pg', '0.12.2'
end

当我在终端中输入'bundle install'时，我收到一条错误消息：

Gemfile syntax error:
group :deve...3.2.7'ygems.org'
...                               ^

有关如何解决这个问题的建议吗？