Fix dead URLs to ftp.kernel.org

URLs to ftp.kernel.org are still exist though the service is closed [0].
This commit fixes the URLs to use www.kernel.org instead.

[0] https://www.kernel.org/shutting-down-ftp-services.html

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index b7767da..585ff32 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -115,7 +115,7 @@
 
 	  RAID-10 requires mdadm-1.7.0 or later, available at:
 
-	  ftp://ftp.kernel.org/pub/linux/utils/raid/mdadm/
+	  https://www.kernel.org/pub/linux/utils/raid/mdadm/
 
 	  If unsure, say Y.