Swap <Shift>/plain <F5> and <F6> in XMonad

Brightness keys.  Before:

Mod + F5, F6 = down up
Mod + Shift + F5, F6 = min max

But I found myself only using `max`, so now:

Mod + F5, F6 = min max
Mod + Shift + F5, F6 = down up