Documentation: PCI: Update cross references to .rst files

Change references to *.txt to *.rst to match the current filenames.

Link: https://lore.kernel.org/r/20230609222500.1267795-4-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>