Fix warning rendering in Export your license usage docs

A blank line between the `WARNING:` and its text broke the rendering of
the red warning box.

This change removes the blank line in order to fix the render issue.

doc/subscriptions/self_managed/index.md

@@ -275,7 +275,6 @@ NOTES:
 - A custom format is used for [dates](https://gitlab.com/gitlab-org/gitlab/blob/3be39f19ac3412c089be28553e6f91b681e5d739/config/initializers/date_time_formats.rb#L7) and [times](https://gitlab.com/gitlab-org/gitlab/blob/3be39f19ac3412c089be28553e6f91b681e5d739/config/initializers/date_time_formats.rb#L13) in CSV files.
 
 WARNING:
-
 Do not open the license usage file. If you open the file, failures might occur when [you submit your license usage data](../../administration/license_file.md#submit-license-usage-data).
 
 ## Renew your subscription