Documentation: KVM: Use unordered list for pre-init VGIC registers

The intent was to create a single column table, however the markup used
was actually for a header which led to docs build failures:

  Sphinx parallel build error:
  docutils.utils.SystemMessage: Documentation/virt/kvm/devices/arm-vgic-v3.rst:128: (SEVERE/4) Unexpected section title or transition.

Fix the issue by converting the attempted table to an unordered list.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250729142217.0d4e64cd@canb.auug.org.au/


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Message-ID: <20250729152242.3232229-1-oliver.upton@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>