Fix dracut command in docs

The current command line works correctly only if one kernel is
installed. It picks the oldest if there are multiple, which is rather
unlikely to be correct. Using "tail" instead of "head" would work also,
but --regenerate-all makes it briefer and safer.

userdocs/contents/boot-management.rst

@@ -161,7 +161,7 @@ initramfs inside the container.
 .. code-block:: shell
 
    dnf -y install warewulf-dracut
-   dracut --force --no-hostonly --add wwinit --kver $(ls /lib/modules | head -n1)
+   dracut --force --no-hostonly --add wwinit --regenerate-all
 
 Set the node's iPXE template to ``dracut`` to direct iPXE to fetch the
 node's initramfs image and boot with dracut semantics, rather than