KVM: Documentation: Update entry for KVM_X86_SET_MSR_FILTER

It should be an accident when rebase, since we've already have section
8.25 (which is KVM_CAP_S390_DIAG318).  Fix the number.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012044.5151-2-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>