Update MacOS installation instructions.
Update MacOS installation instructions.

Homebrew/homebrew-cask#55358 changed the name used for installation.
resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher,resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/resin-etcher