Fix formatting for deprecation notice

What does this MR do and why?

Fixes formatting for the deprecation notice at the top of https://docs.gitlab.com/ee/user/project/integrations/slack.html by removing the extra line break.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.