tap: tweak core tap warning

Most users don't need the core taps to be installed locally but the
current message doesn't communicate that they're sometimes necessary
for local development. This just tweaks the message to be more
informative.

```console
$ brew tap homebrew/cask
Error: Tapping homebrew/cask is no longer typically necessary.
Add --force if you are sure you need it for local development.
```

Library/Homebrew/tap.rb

@@ -434,7 +434,7 @@ class Tap
       return
     elsif (core_tap? || core_cask_tap?) && !Homebrew::EnvConfig.no_install_from_api? && !force
       odie "Tapping #{name} is no longer typically necessary.\n" \
-           "Add #{Formatter.option("--force")} if you are sure you need it done."
+           "Add #{Formatter.option("--force")} if you are sure you need it for local development."
     end
 
     clear_cache