missing_formula: fix cask installation advice on tap migration

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.

Library/Homebrew/missing_formula.rb

@@ -109,7 +109,7 @@ module Homebrew
           break if new_tap_name == CoreTap.instance.name
 
           install_cmd = if new_tap_name.start_with?("homebrew/cask")
-            "cask install"
+            "install --cask"
           else
             "install"
           end