MAINTAINERS: keep UBI and UBIFS stuff in the same tree

These 2 subsystems are so much related, and the traffic there became rather
small, so that maintaining 2 separate git trees adds no value, just burden.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

MAINTAINERS

@@ -8888,7 +8888,7 @@ UNSORTED BLOCK IMAGES (UBI)
 M:	Artem Bityutskiy <dedekind1@gmail.com>
 W:	http://www.linux-mtd.infradead.org/
 L:	linux-mtd@lists.infradead.org
-T:	git git://git.infradead.org/ubi-2.6.git
+T:	git git://git.infradead.org/ubifs-2.6.git
 S:	Maintained
 F:	drivers/mtd/ubi/
 F:	include/linux/mtd/ubi.h