readme: small update to installing on Ubuntu section
readme: small update to installing on Ubuntu section

Signed-off-by: Mihai Tudor Panu <a27de6130c5d32f5817bf40bb8b348582f37a8af@intel.com>

KurtE/mraa,malikabhi05/mraa,malikabhi05/mraa,Propanu/mraa,stefan-andritoiu/mraa,intel-iot-devkit/mraa,sergev/mraa,yongli3/mraa,alext-mkrs/mraa,Propanu/mraa,malikabhi05/mraa,stefan-andritoiu/mraa,malikabhi05/mraa,intel-iot-devkit/mraa,malikabhi05/mraa,yongli3/mraa,yongli3/mraa,arfoll/mraa,g-vidal/mraa,intel-iot-devkit/mraa,g-vidal/mraa,yongli3/mraa,yongli3/mraa,arfoll/mraa,Propanu/mraa,alext-mkrs/mraa,alext-mkrs/mraa,alext-mkrs/mraa,sergev/mraa,sergev/mraa,KurtE/mraa,g-vidal/mraa,intel-iot-devkit/mraa,g-vidal/mraa,g-vidal/mraa,arfoll/mraa,stefan-andritoiu/mraa,intel-iot-devkit/mraa,sergev/mraa,KurtE/mraa,stefan-andritoiu/mraa,arfoll/mraa,stefan-andritoiu/mraa,KurtE/mraa,arfoll/mraa,alext-mkrs/mraa,KurtE/mraa,Propanu/mraa,Propanu/mraa,sergev/mraa