sysfs.txt: fix pre-kernfs sysfs_dirent reference

sysfs_dirent went away when kernfs was extracted from sysfs. The reference
to the kobject now lives in a kernfs_node (in the 'priv' member).

See commit 324a56e1

 ("kernfs: s/sysfs_dirent/kernfs_node/ and rename
its friends accordingly").

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>