MAINTAINERS: Update git tree for mdraid subsystem

Moving the official git tree to the MDRAID Group account.

Signed-off-by: Song Liu <song@kernel.org>

Author: liu-song-6

MAINTAINERS

@@ -21307,7 +21307,7 @@ M:	Yu Kuai <yukuai3@huawei.com>
 L:	linux-raid@vger.kernel.org
 S:	Supported
 Q:	https://patchwork.kernel.org/project/linux-raid/list/
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/song/md.git
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mdraid/linux.git
 F:	drivers/md/Kconfig
 F:	drivers/md/Makefile
 F:	drivers/md/md*