Fix headings

doc/user/admin_area/user_cohorts.md

 # Cohorts
 
 > **Notes:**
-- [Introduced][ce-23361] in GitLab 9.1.
+> [Introduced][ce-23361] in GitLab 9.1.
 
 As a benefit of having the [usage ping active](settings/usage_statistics.md),
 GitLab lets you analyze the users' activities of your GitLab installation.
 Under `/admin/cohorts`, when the usage ping is active, GitLab will show the
 monthly cohorts of new users and their activities over time.
 
+## Overview
+
 How do we read the user cohorts table? Let's take an example with the following
 user cohorts.
 
@@ -23,12 +25,13 @@ The Inactive users column shows the number of users who have been added during
 the month, but who have never actually had any activity in the instance.
 
 How do we measure the activity of users? GitLab considers a user active if:
+
 * the user signs in
 * the user has Git activity (whether push or pull).
 
-### Setup
+## Setup
 
-1. Activate the usage ping as defined [in the documentation](settings/usage_statistics.md)
+1. [Activate the usage ping](settings/usage_statistics.md)
 2. Go to `/admin/cohorts` to see the user cohorts of the server
 
 [ce-23361]: https://gitlab.com/gitlab-org/gitlab-ce/issues/23361