MAINTAINERS: adjust to kobject doc ReST conversion

Commit 5fed00dc ("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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

MAINTAINERS

@@ -5201,7 +5201,7 @@ M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 R:	"Rafael J. Wysocki" <rafael@kernel.org>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git
 S:	Supported
-F:	Documentation/kobject.txt
+F:	Documentation/core-api/kobject.rst
 F:	drivers/base/
 F:	fs/debugfs/
 F:	fs/sysfs/