Fix tier badge code

Fix tier badge to add parentheses.

doc/administration/maintenance_mode/index.md

@@ -4,7 +4,7 @@ group: Geo
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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-# GitLab in maintenance mode **PREMIUM SELF**
+# GitLab in maintenance mode **(PREMIUM SELF)**
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2149) in GitLab Premium 13.9.