MAINTAINERS: update MTD list

Trying to send an email to mtd@infradead.org gives you the answer that 
you should use linux-mtd@lists.infradead.org instead.

So let's update the entry in MAINTAINERS.
Signed-off-by: Adrian Bunk <bunk@fs.tum.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

MAINTAINERS

@@ -1374,7 +1374,7 @@ MEMORY TECHNOLOGY DEVICES
 P:	David Woodhouse
 M:	dwmw2@redhat.com
 W:	http://www.linux-mtd.infradead.org/
-L:	mtd@infradead.org
+L:	linux-mtd@lists.infradead.org
 S:	Maintained
 
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