README: remove version flag from build commands

The firmware version is set in the `prj.conf` file. Remove the command line
flag from the nrf9160dk sample build commands to align with this approach.

Signed-off-by: Mike Szczys <[email protected]>

Author: szczys

README.rst

@@ -151,12 +151,13 @@ Nordic nRF9160 DK
 
 This reference design may be built for the `Nordic nRF9160 DK`_.
 
-Use the following commands to build and program. (Use the same console commands
-from above to provision this board after programming the firmware.)
+Prior to building, update ``CONFIG_MCUBOOT_IMGTOOL_SIGN_VERSION`` in the ``prj.conf`` file to
+reflect the firmware version number you want to assign to this build. Then run the following
+commands to build and program the firmware onto the device.
 
 .. code-block:: text
 
-   $ (.venv) west build -p -b nrf9160dk_nrf9160_ns app -- -DCONFIG_MCUBOOT_IMGTOOL_SIGN_VERSION=\"<your.semantic.version>\"
+   $ (.venv) west build -p -b nrf9160dk_nrf9160_ns app
    $ (.venv) west flash
 
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