2e6422444894 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
incorrectly renamed PCIEBUS-HOWTO.txt to picebus-howto.rst.

Rename it to pciebus-howto.rst.

Fixes: 2e6422444894 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
Signed-off-by: Bjorn Helgaas