MAINTAINERS: adjust to kobject doc ReST conversion

MAINTAINERS: adjust to kobject doc ReST conversion

Commit 5fed00dcaca8 ("Documentation: kobject.txt has been moved to
core-api/kobject.rst") missed to adjust the entry in MAINTAINERS.

Since then, ./scripts/get_maintainer.pl --self-test complains:

  warning: no file matches F: Documentation/kobject.txt

Adjust DRIVER CORE, KOBJECTS, DEBUGFS AND SYSFS entry in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200304110821.7243-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>