Wrong link to brew github repo on the Marta Download homepage

[karczewsky]

On the Marta download page, there is a wrong link to the Brew Github repo.

Change required: [brew](https://brew.sh/) install --cask [marta](https://github.com/Homebrew/homebrew-cask/blob/master/Casks/marta.rb) To: [brew](https://brew.sh/) install --cask [marta](https://github.com/Homebrew/homebrew-cask/blob/master/Casks/m/marta.rb)