Fix example code (rubocop#212)

See http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-update_attribute

README.md

@@ -553,7 +553,7 @@ render status: :forbidden
   person.toggle :active
   product.touch
   Billing.update_all("category = 'authorized', author = 'David'")
-  user.update_attribute(website: 'example.com')
+  user.update_attribute(:website, 'example.com')
   user.update_columns(last_request_at: Time.current)
   Post.update_counters 5, comment_count: -1, action_count: 1