Update brew documentation for installation

[mikolayek]

README.md contains a shell command to install blurred.

When I run it, I am getting an error:

 brew cask install blurred
Error: `brew cask` is no longer a `brew` command. Use `brew <command> --cask` instead.

[lwu877]

This is because brew has updated since they've wrote the README.

The real command should be the following: brew install --cask blurred

[hyperjeff]

I just did "brew install blurred" and it worked fine, fwiw.