md: update MAINTAINERS info

I have been reviewing patches for md in the past few months. Mark me
as the MD maintainer, as I have effectively been filling that role.

Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -14882,9 +14882,9 @@ F:	include/linux/arm_sdei.h
 F:	include/uapi/linux/arm_sdei.h
 
 SOFTWARE RAID (Multiple Disks) SUPPORT
-M:	Shaohua Li <shli@kernel.org>
+M:	Song Liu <song@kernel.org>
 L:	linux-raid@vger.kernel.org
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/shli/md.git
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/song/md.git
 S:	Supported
 F:	drivers/md/Makefile
 F:	drivers/md/Kconfig