bug#49691: `brew cask install` should be `brew install --cask` (Emacs website)

[tsuucat]

Hi,

current Emacs website[1] says `brew cask install emacs` as install
command on MacOS but `brew cask ...` is now deprecated[2].
`brew install --cask emacs` should be used instead of `brew cask
install emacs`.

Thanks.

[1]: https://www.gnu.org/software/emacs/download.html
[2]: https://brew.sh/2020/12/01/homebrew-2.6.0/

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tsuucat