Documentation: KVM: update s390-diag.rst reference

Changeset daec8d40 ("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: daec8d40 ("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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/virt/kvm/x86/hypercalls.rst

@@ -22,7 +22,7 @@ S390:
   number in R1.
 
   For further information on the S390 diagnose call as supported by KVM,
-  refer to Documentation/virt/kvm/s390-diag.rst.
+  refer to Documentation/virt/kvm/s390/s390-diag.rst.
 
 PowerPC:
   It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.