updating brew command from docs

[balexandre]

a quick fix on the documentation so we don't get someone that is new to brew wondering

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

with the right command

❯ brew install telegram --cask
==> Downloading https://formulae.brew.sh/api/cask.jws.json
...

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1 out of 3 committers have signed the CLA.

:white_check_mark: balexandre
:x: Mike Renoir
:x: overtake

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