Documentation: kvm: extend KVM_S390_ZPCI_OP subheading underline

Stephen Rothwell reported the htmldocs warning:

Documentation/virt/kvm/api.rst:5959: WARNING: Title underline too short.

4.137 KVM_S390_ZPCI_OP
--------------------

The warning is due to subheading underline on KVM_S390_ZPCI_OP section is
short of 2 dashes.

Extend the underline to fix the warning.

Link: https://lore.kernel.org/linux-next/20220711205557.183c3b14@canb.auug.org.au/
Fixes: a0c4d1109d6cc5 ("KVM: s390: add KVM_S390_ZPCI_OP to manage guest zPCI devices")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Pierre Morel <pmorel@linux.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Matthew Rosato <mjrosato@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: kvm@vger.kernel.org
Cc: linux-s390@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220712092954.142027-4-bagasdotme@gmail.comSigned-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>

Documentation/virt/kvm/api.rst

@@ -5956,7 +5956,7 @@ KVM_PV_DUMP_CPU
   The length of the returned data is provided by uv_info.guest_cpu_stor_len.
 
 4.137 KVM_S390_ZPCI_OP
---------------------
+----------------------
 
 :Capability: KVM_CAP_S390_ZPCI_OP
 :Architectures: s390