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/Fedora/Root on ZFS/3-system-configuration.rst

@@ -55,7 +55,7 @@ System Configuration
     for i in $m; do mount --rbind $i /mnt/$i; done
 
     history -w /mnt/home/sys-install-pre-chroot.txt
-    chroot /mnt /usr/bin/env DISK=$DISK bash --login
+    chroot /mnt /usr/bin/env DISK="$DISK" bash --login
 
 #. For SELinux, relabel filesystem on reboot::