paths were wrong, tested in mac, not on windows

[jacobrosenthal]

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!

[cmaglie]

IMHO removing the trailing slash from WIFI_FW_PATH and AVR_TOOLS_PATH is not correct since it is composed as:

ARDUINO_PATH=$OPTARG
WIFI_FW_PATH=$ARDUINO_PATH$WIFI_FW_PATH
AVR_TOOLS_PATH=$ARDUINO_PATH$AVR_TOOLS_PATH

if the command line argument in OPTARG doesn't have the final / we may end with a WIFI_FW_PATH like /my/home/folderhardware/arduino/firmware/.... On the other end the double // is tolerated.

The wrong path on _mac script instead is already covered with #2118

Thank you for the pull request