Update lint example to configure DatabaseCleaner

Not specifying `strategy`/`clean_with` can lead to test data sticking
around on CI and cause unique validations to fail while the lint
process is running.

GETTING_STARTED.md

@@ -1003,6 +1003,7 @@ namespace :factory_bot do
   desc "Verify that all FactoryBot factories are valid"
   task lint: :environment do
     if Rails.env.test?
+      DatabaseCleaner.clean_with(:deletion)
       DatabaseCleaner.cleaning do
         FactoryBot.lint
       end