docs: vfio: fix header path

The text points to a different header file, fix by changing
the path to "uapi".

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230310095857.985814-1-jiri@resnulli.us


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