Fix app/controllers/instructor/sections_controller.rb to work with AJAX

Hey Nan -

Here are the changes you need to make to fix your app.  Descriptions inline.

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Hope this helps.

Aloha,
Ken

We're performing an update, so we should update the "current_section" that we look up by the `params[:id]`  a course can have many sections so using `current_course.section` doesn't work.