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.