Documentation: KVM: update s390-diag.rst reference

Documentation: KVM: update s390-diag.rst reference

Changeset daec8d408308 ("Documentation: KVM: add separate directories for architecture-specific documentation")
renamed: Documentation/virt/kvm/s390-diag.rst
to: Documentation/virt/kvm/s390/s390-diag.rst.

Update its cross-reference accordingly.

Fixes: daec8d408308 ("Documentation: KVM: add separate directories for architecture-specific documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/85b81e4678bbe23d0e9692616798762a6465f0a3.1656234456.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>