Update docs for installing wp-cli via homebrew

[Konstantin Obenland]

From https://meta.trac.wordpress.org/ticket/3629:

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

The following command no longer works as homebrew/php is now 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.

[Alain Schlesser]

Thanks for moving the issue into GitHub, @obenland!