fix(single-stat): Add left margin to single stat unit

mentioned in commit 831bbabd

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!104 (merged)

mentioned in merge request status-page!659 (merged)

mentioned in merge request gitlab!106497 (merged)

mentioned in merge request gitlab-docs!3354 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3180 (merged)

mentioned in merge request gitlab-docs!3355 (closed)