Documentation/PCI/pci.txt: fix a reference doc name

Documentation/PCI/pci.txt: fix a reference doc name

IO-mapping.txt was renamed to "bus-virt-phys-mapping.txt",
it does only contain a tiny reference to the ioremap(),
the best reference is to Documentation/DocBook/deviceiobook.tmpl,
which is all about accessing device registers.

Changes from v1:
- change reference doc to "Documentation/DocBook/deviceiobook.tmpl"

Signed-off-by: Amos Kong <kongjianjun@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>