docs: virt: kvm: s390-pv-boot.rst: avoid using ReST :doc:foo markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8c0fc6578ff6384580fd0d622f363bbbd4fe91da.1623824363.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mchehab

Documentation/virt/kvm/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
-:doc:`s390-pv` for details."
+Documentation/virt/kvm/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