Fix line-height of help text in radios/checkboxes

Similar to #585 (closed), the line-height is being inherited and not set directly.

Current	Specs
https://design.gitlab.com/components/radio-button/ and https://design.gitlab.com/components/checkbox	https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/radiobuttons-spec-previews/