Documentation/initrd.txt: Change the location of util-linux

The address of util-linux seems deprecated. The new util-linux location is
in the kernel.org. So, change this for the correct address.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>