fix Chapter 16's extra 'select' in the instruction

[stevechen]

This EdgeQL code causes an error for place_name in select (array_unpack(Place.important_places)) union (insert Landmark { name := place_name });

removing the 'select' fixes it.

[stevechen]

I think the parentheses around the array_unpack are also extraneous if you want to remove those, too.

First time doing a pull request… so not sure what's the best way to do that. Just make another commit and push?