Fix HAML in layouts/nav/_breadcrumbs.html.haml

What does this MR do?

Fixes HAML in app/views/layouts/nav/_breadcrumbs.html.haml to follow Layout/SpaceAfterColon.

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