Update description for show_exceptions in environments/test.rb

In e28f147 the `show_exceptions`
default was updated to `:rescuable` in environments/test.rb.
The description still described the old default value (`false`), instead
of `:rescuable`.

Author: p8

railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt

@@ -27,7 +27,7 @@ Rails.application.configure do
   config.action_controller.perform_caching = false
   config.cache_store = :null_store
 
-  # Raise exceptions instead of rendering exception templates.
+  # Render exception templates for rescuable exceptions and raise for other exceptions.
   config.action_dispatch.show_exceptions = :rescuable
 
   # Disable request forgery protection in test environment.