Documentation: kvm: fix warning

Documentation/virt/kvm/api.rst:4927: WARNING: Title underline too short.

4.130 KVM_XEN_VCPU_GET_ATTR
--------------------------

Fixes: e1f68169 ("KVM: Add documentation for Xen hypercall and shared_info updates")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -4924,7 +4924,7 @@ KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO
   for a given vCPU. This is typically used for guest vsyscall support.
 
 4.130 KVM_XEN_VCPU_GET_ATTR
---------------------------
+---------------------------
 
 :Capability: KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_SHARED_INFO
 :Architectures: x86