docs: PCI: endpoint: pci-endpoint-cfs.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`.

Acked-by: Bjorn Helgaas <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/4b18fe[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/PCI/endpoint/pci-endpoint-cfs.rst

···
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-[1] :doc:`pci-endpoint`
+[1] Documentation/PCI/endpoint/pci-endpoint.rst