hammerspoon: fix precedence in screen event handling
hammerspoon: fix precedence in screen event handling

I wondered why this wasn't working properly and it turns out that I
can't Lua:

    > 1 == 2
    false

    > not 1 == 2
    false

    > not (1 == 2)
    true

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent