Fix Guide HTML validation

Hi!

The [Ruby on Rails Guides Guidelines](http://edgeguides.rubyonrails.org/ruby_on_rails_guides_guidelines.html) say that you should make sure all guides are valid HTML and that `bundle exec rake guides:generate` should be run with `WARNINGS=1` to make sure our internal links are valid. To my surprise, there were quite a few validation errors!

Now, `rake guides:validate` returns no errors and `rake guides:generate WARNINGS=1` returns no warnings.

I left these commits small and separate, please instruct if you wish me to squash them.

Expect a PR soon moving the guides to HTML5, hopefully it won't be too much work!