When an installed formula is missing and its source has migrated
to a new tap, Homebrew warns the user about this, saying:

You can access it again by running:
  brew tap new-name
And then you can install it by running:
  brew cask install new-name

Unfortunately, the "brew cask install" incantation is deprecated
and the advice won't work:

Error: Calling brew cask install is disabled! Use brew install [--cask] instead.

This alters the advice to use the new "brew install --cask" notation,
fixing the advice presented to the user.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?