fix: use valid url syntax in form_checkbox.scss

mentioned in commit 854881b3

mentioned in merge request status-page!600 (merged)

mentioned in merge request gitlab!95922 (merged)

mentioned in merge request gitlab-docs!3063 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3030 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!87 (merged)