chore(docs): add missing prefix to brew command

[ayewo]

The README is has a typo in the instructions for installation via brew which was merged last month:

brew install copilot-cli

It is trying to install a non-existent formula copilot-cli (404) instead of via cask copilot-cli:

Warning: No available formula with the name "copilot-cli". Did you mean copilot?
==> Searching for similarly named formulae and casks...
==> Formulae
copilot

To install copilot, run:
  brew install copilot

==> Casks
copilot-for-xcode

To install copilot-for-xcode, run:

This PR fixes that. Actually, I take that back.

Running the fix: brew install --cask copilot-cli added in my PR doesn't work on my machine (macOS Sequoia 15.6):

 brew install --cask copilot-cli
Warning: Cask 'copilot-cli' is unavailable: No Cask with this name exists.
==> Searching for similarly named casks...
==> Casks
copilot-for-xcode

To install copilot-for-xcode, run:
  brew install --cask copilot-for-xcode

Had to go with pnpm i -g @github/copilot for the time being.

[talledoscarlos8-tech]

Todo Bien

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IA eStudio Talledos

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