[Gimp-web] Improve "Gimp for mac" page

Hi!
In this page
https://www.gimp.org/downloads/Mac.html
told that brew have no cask for it. Now it have!
Look at
https://github.com/caskroom/homebrew-cask/blob/master/Casks/lisanet-gimp.rb

It can be installed via `brew cask install lisanet-gimp`