README.md: Updated flash instruction links
README.md: Updated flash instruction links

Updated the flash instructions per BOARD target for the shell sample.

Signed-off-by: Noel Eck <1e36ab6ededef40f3f16bdbc1b09eee72c5d08cf@intel.com>

malikabhi05/zmraa-1,intel-iot-devkit/zmraa,jontrulson/zmraa