cmd_ext4: remove TABs from command help text

TABs in the help text won't line up in the same place on the console as
in a source editor. Replace them with spaces to make ensuring correct
alignment easier.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>