Documentation PCI: Fix pciebus-howto.rst filename typo

2e642244 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
incorrectly renamed PCIEBUS-HOWTO.txt to picebus-howto.rst.

Rename it to pciebus-howto.rst.

Fixes: 2e642244

 ("Documentation: PCI: convert PCIEBUS-HOWTO.txt to reST")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>