paths were wrong, tested in mac, not on windows

The firmware upgrade script has bad paths


```

       Arduino WiFi Shield upgrade
=========================================
Disclaimer: to access to the USB devices correctly, the dfu-programmer needs to be used as root. Run this script as root.

./avr-objcopy: '/Applications/Arduino.app/Contents/Resources/Java//hardware/arduino/firmwares/wifi-shield/wifi_dnld.elf': No such file
./avr-objcopy: '/Applications/Arduino.app/Contents/Resources/Java//hardware/arduino/firmwares/wifi-shield/wifiHD.elf': No such file
****Upgrade WiFi Shield firmware****

Error opening the file.
Something went wrong with creating the memory image.

Done. Remove the J3 jumper and press the RESET button on the shield.
Thank you!

```