It's likely that you can't right now with those instructions, and that the formula as it currently is writtent is broken for installing the putty executable. I'd recommend not doing so at all. Have a quick read of https://github.com/Homebrew/homebrew-core/issues/1078 for more info.
You can install plink, pscp, psftp and puttygen using the current formula, but not the putty gui.
As an aside, for the most part you're probably better off with using openssh (
brew install openssh) for most of the things that putty would have given you.