mraa/joule: Update the documentation on SPI
mraa/joule: Update the documentation on SPI

The existing documentation shows the MISO and MOSI
pins wrongly. As per the hardware document available
at http://www.intel.com/content/dam/support/us/en/documents/
joule-products/intel-joule-dev-kit-hardware-guide.pdf
pin2 should be MISO and pin 4, MOSI.

Signed-off-by: Arun Ravindran <34b1a85c2c1536c8cbda227811bb4d19da7bcbf1@intel.com>

arunlee77/mraa,arunlee77/mraa,arunlee77/mraa,arunlee77/mraa