Change `Topic.new(params[:topic])` to `Topic.new(topic_params)` 

Noticed error in the ['Redirect to the Topics List...'](http://curriculum.railsbridge.org/curriculum/redirect_to_the_topics_list_after_creating_a_new_topic) page. It says to use `param[:topic]` instead of the user defined controller method to handle user input as required by strong parameters (`topic_params`). Since we're using Rails 4 these days with strong_parameters this is going to break and cause some confusion.