Fix HAML in admin/broadcast_messages/_form.html.haml

What does this MR do?

Fixes HAML in admin/broadcast_messages/_form.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" and ~"feature::maintenance" .
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