mmdebstrap: Use ext4 instead of ext2 in examples

mmdebstrap

@@ -6821,7 +6821,7 @@ Create a bootable USB Stick that boots into a full Debian desktop:
         part-disk /dev/sda mbr : \
         part-set-bootable /dev/sda 1 true : \
         set-label /dev/sda1 rootfs : \
-        mkfs ext2 /dev/sda1 : mount /dev/sda1 / : \
+        mkfs ext4 /dev/sda1 : mount /dev/sda1 / : \
         tar-in debian-unstable.tar / xattrs:true : \
         upload /usr/lib/SYSLINUX/mbr.bin /mbr.bin : \
         copy-file-to-device /mbr.bin /dev/sda size:440 : rm /mbr.bin : \
@@ -6859,7 +6859,7 @@ Use as replacement for autopkgtest-build-qemu and vmdb2:
     $ guestfish -N debian-unstable.img=disk:8G -- \
         part-disk /dev/sda mbr : \
         part-set-bootable /dev/sda 1 true : \
-        mkfs ext2 /dev/sda1 : mount /dev/sda1 / : \
+        mkfs ext4 /dev/sda1 : mount /dev/sda1 / : \
         tar-in debian-unstable.tar / xattrs:true : \
         extlinux / : copy-in extlinux.conf / : sync : umount / : shutdown
     $ qemu-img convert -O qcow2 debian-unstable.img debian-unstable.qcow2