Merge branch 'docs-background-migrations-cleanup' into 'master'

Docs: Clean up background migration checking Ruby snippets

See merge request gitlab-org/gitlab!53877

doc/update/index.md

@@ -103,7 +103,7 @@ sudo -u git -H bundle exec rails runner -e production 'puts Gitlab::BackgroundMi
 If using GitLab 12.8 and older, run the following using a [Rails console](../administration/operations/rails_console.md#starting-a-rails-console-session):
 
 ```ruby
-puts Sidekiq::Queue.new("background_migration").size
+Sidekiq::Queue.new("background_migration").size
 Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
 ```