package/mdadm: update website link

Neil Brown no longer maintains mdadm. The old website refers to a stale
git repository. There is nothing else but this wiki page to serve as a
website.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/mdadm/Config.in

@@ -5,7 +5,7 @@ config BR2_PACKAGE_MDADM
 	help
 	  Utility for managing Linux Software RAID arrays.
 
-	  http://neil.brown.name/blog/mdadm
+	  https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm
 
 comment "mdadm needs a toolchain w/ dynamic library"
 	depends on BR2_STATIC_LIBS