Remove mention of update_attribute being deprecated and removed

The method was removed about 2 years ago, but that change has since been reverted: https://github.com/rails/rails/pull/6738#issuecomment-39584005

I also linked the method to its documentation on the RoR API docs: http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-update_attribute