Add double quotes to DISK=$DISK

Without the double quotes, the $DISK variable expands and will not
handle multi-disk correctly.

Co-authored-by: @baberlevi

Signed-off-by: Maurice Zhou <ja@apvc.uk>

docs/Getting Started/Alpine Linux/Root on ZFS/2-system-installation.rst

@@ -157,7 +157,7 @@ System Installation
     m='/dev /proc /sys'
     for i in $m; do mount --rbind $i /mnt/$i; done
 
-    chroot /mnt /usr/bin/env DISK=$DISK sh
+    chroot /mnt /usr/bin/env DISK="$DISK" sh
 
 #. Rebuild initrd::