doc: add dfu-util to Linux dependencies

Since we recommend using dfu-util to flash the Arduino 101 board,
it should be mentioned in the list of packages to be installed.
Also fix indentation.

Change-Id: Iafc053c1b07e79bd7d02059b46a842c25ebaaf64
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>