Add a missing RAILS_ENV=production to crontab clearing

All other commands in the guide set this env var, but it's missing here
and this causes whenever not to clear the crontab file of gitab_ci
properly.

doc/migrate_ci_to_ce/README.md

@@ -143,7 +143,7 @@ sudo gitlab-ctl stop ci-sidekiq
 # Source
 sudo service gitlab_ci stop
 cd /home/gitlab_ci/gitlab-ci
-sudo -u gitlab_ci -H bundle exec whenever --clear-crontab
+sudo -u gitlab_ci -H bundle exec whenever --clear-crontab RAILS_ENV=production
 ```
 
 ### II. Moving data