Fixed list_display to match screenshot and fixed double registration err...

The order of arguments in list_display is not matching the order of the screenshot right below. list_display('category', 'title', 'url') is the input. However, the screenshot below shows the order of display to be title, category and url.

Instruction says to add the extra line ``admin.site.register(Page, PageAdmin)`` to the current admin.py. However, this yields "the model "Page" is already registered" error since a registration with model Page already exists from the previous steps of the tutorial. Instead of adding a new line, we should modify the current line ``admin.site.register(Page)`` to add PageAdmin to its argument such as ``admin.site.register(Page, PageAdmin)``

By the way, this is amazing work! Thank you for such an awesome complete beginner-friendly tutorial, it was much needed for the Django community. This is the best on the internet. Keep it up!