bug#46700: Update instructions for mac OS installation of emacs via homebrew

[Eli Zaretskii]

> Date: Sun, 21 Feb 2021 15:48:46 -0500
> From: "Tyler Chambers" <tyler@iheartapis.com>
> 
> on the page:
> https://www.gnu.org/software/emacs/download.html
> 
> brew cask install emacs
> 
> should be replaced with:
> 
> brew install --cask emacs
> 
> as the cask subcommand has been removed from homebrew, in favor of a --cask 
> flag.
> 
> relevant commit removing the cask subcommand from the usage instructions on 
> the official homebrew
> github:
> https://github.com/Homebrew/homebrew-cask/commit/1fb821a4a50f75c92afe1ded44ccc6709314461b#diff-58a825c70bccfbe691d7ddbddb133275f4f657de6aeb4a502fa9fce279f8bf8d

Alan, any comments?  Should this be changed, and if so, should we do
it on the release branch?

Thanks.