Fix capitalization in docs for works like GitLab, Git
Omnibus, etc

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

https://community.atlassian.com/t5/Jira-questions/Is-it-quot-JIRA-quot-or-quot-Jira-quot/qaq-p/681163Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>