Merge branch 'unnececssary-slack-redirect' into 'master'

Documentation - Removed the unnecessary loop

See merge request gitlab-org/gitlab-ce!31003

doc/integration/slack.md

 ---
-redirect_to: '../project_services/slack.md'
+redirect_to: '../user/project/integrations/slack.md'
 ---
 
-This document was moved to [project_services/slack.md](../project_services/slack.md).
+This document was moved to [project_services/slack.md](../user/project/integrations/slack.md).