Documentation: KVM: update s390-pv.rst reference

Changesets: daec8d40 ("Documentation: KVM: add separate directories for architecture-specific documentation")
and: daec8d40 ("Documentation: KVM: add separate directories for architecture-specific documentation")
renamed: Documentation/virt/kvm/s390-pv.rst
to: Documentation/virt/kvm/s390/s390-pv.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/e2676f087d287db0bc31ae7c05c80ce5adf93333.1656234456.git.mchehab@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/virt/kvm/s390/s390-pv-boot.rst

@@ -10,7 +10,7 @@ The memory of Protected Virtual Machines (PVMs) is not accessible to
 I/O or the hypervisor. In those cases where the hypervisor needs to
 access the memory of a PVM, that memory must be made accessible.
 Memory made accessible to the hypervisor will be encrypted. See
-Documentation/virt/kvm/s390-pv.rst for details."
+Documentation/virt/kvm/s390/s390-pv.rst for details."
 
 On IPL (boot) a small plaintext bootloader is started, which provides
 information about the encrypted components and necessary metadata to