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>
diff --git a/doc/getting_started/installation_linux.rst b/doc/getting_started/installation_linux.rst
index 94ebfda..2d67e0b 100644
--- a/doc/getting_started/installation_linux.rst
+++ b/doc/getting_started/installation_linux.rst
@@ -55,7 +55,7 @@
 .. code-block:: console
 
    $ sudo apt-get install git make gcc g++ python3-ply ncurses-dev \
-   	python-yaml python2
+	 python-yaml python2 dfu-util
 
 Install the required packages in a Fedora host system with:
 
@@ -64,7 +64,7 @@
    $ sudo dnf group install "Development Tools"
    $ sudo dnf install git make gcc glibc-static \
 	 libstdc++-static python3-ply ncurses-devel \
-	 python-yaml python2
+	 python-yaml python2 dfu-util
 
 .. _zephyr_sdk: