Fix HAML in projects/services/gitlab_slack_application/_slack_button.html.haml

What does this MR do?

Fixes HAML in ee/app/views/projects/services/gitlab_slack_application/_slack_button.html.haml simply to comply Layout/SpaceAfterColon in HAML-Lint.

Part of #326884 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because ~"technical debt".
• Code review guidelines
• Style guides
• Separation of EE specific content