Documentation: fix firewire.rst ABI file path error

Adjust the path of the ABI files for firewire.rst to prevent a
documentation build error. Prevents this problem:

Sphinx parallel build error:
docutils.utils.SystemMessage: Documentation/driver-api/firewire.rst:22: (SEVERE/4) Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: '../Documentation/driver-api/ABI/stable/firewire-cdev'.

Fixes: 2f4830ef ("FireWire: add driver-api Introduction section")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220119033905.4779-1-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>