Update docs for installing wp-cli via homebrew

[JoeFusco]

​https://make.wordpress.org/cli/handbook/installing/#installing-via-homebrew

The following command no longer works as homebrew/php is no deprecated:

brew install homebrew/php/wp-cli

WP-CLI can now be installed simply with:

brew install wp-cli

Link to formula also needs updating.

[obenland]

I opened ​an issue on the wp-cli GitHub with your request. Feel free to open a PR there with your proposed changes.