Documentation: KVM: Fix title level for PSCI_SUSPEND

The htmldoc build breaks in a funny way with:

<quote>
Sphinx parallel build error:
docutils.utils.SystemMessage: /home/sfr/next/next/Documentation/virt/kvm/api.rst:6175: (SEVERE/4) Title level inconsistent:

For arm/arm64:
^^^^^^^^^^^^^^
</quote>

Swap the ^^s for a bunch of --s...
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>

Documentation/virt/kvm/api.rst

@@ -6047,7 +6047,7 @@ Valid values for 'type' are:
    the VM.
 
 For arm/arm64:
-^^^^^^^^^^^^^^
+--------------
 
    KVM_SYSTEM_EVENT_SUSPEND exits are enabled with the
    KVM_CAP_ARM_SYSTEM_SUSPEND VM capability. If a guest invokes the PSCI