MAINTAINERS: update file entry for HSI subsystem

The HSI documentation was moved into Documentation/driver-api/hsi.rst in
commit 5e995786850e ("docs: split up serial-interfaces.rst"). Update the
corresponding file entry in MAINTAINERS.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

MAINTAINERS

@@ -6005,7 +6005,7 @@ M:	Sebastian Reichel <sre@kernel.org>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/sre/linux-hsi.git
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-hsi
-F:	Documentation/device-drivers/serial-interfaces.rst
+F:	Documentation/driver-api/hsi.rst
 F:	drivers/hsi/
 F:	include/linux/hsi/
 F:	include/uapi/linux/hsi/