docs: filesystems: replace to_dev() with kobj_to_dev()

Commit a4232963 ("driver-core: Move kobj_to_dev from genhd.h to device.h")
introduced kobj_to_dev() function.

Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20200830144135.6956-1-efremov@linux.com


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