USAGE.md: fix uses of /opt (#21879)

USAGE.md

@@ -42,7 +42,7 @@ The command `brew cask install` accepts a Cask token as returned by `brew cask s
 $ brew cask install google-chrome
 ==> Downloading https://dl.google.com/chrome/mac/stable/GGRO/googlechrome.dmg
 ==> Moving App 'Google Chrome.app' to '/Applications/Google Chrome.app'
-🍺  google-chrome staged at '/opt/homebrew-cask/Caskroom/google-chrome/latest' (3 files, 288K)
+🍺  google-chrome staged at '/usr/local/Caskroom/google-chrome/latest' (3 files, 288K)
 ```
 
 ## Uninstalling Casks
@@ -174,8 +174,7 @@ $ brew cask install caskroom/fonts/font-symbola
 You can also modify the default installation locations used when issuing `brew cask install`:
 
 * `--caskroom=/my/path` determines where the actual applications will be located.
-Should be handled with care — setting it outside `/opt` or your home directory might mess up your system.
-Default is `/opt/homebrew-cask/Caskroom`
+Default is `$(brew --repository)/Caskroom`
 * `--appdir=/my/path` changes the path where the applications (above)
 will be moved. Default is `/Applications`.
 * `--prefpanedir=/my/path` changes the path for PreferencePanes.