Fix warning rendering in Export your license usage docs

What does this MR do and why?

Related to Add warning about opening the license usage file (!142194 - merged)

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.

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Screenshots or screen recordings

Current rendering at the end of this section (notice the empty red box): https://docs.gitlab.com/ee/subscriptions/self_managed/#export-your-license-usage (current code)

Expected rendering as seen in https://docs.gitlab.com/ee/administration/license_file.html#license-commands-in-the-rails-console (current code)