Looks like there is a typo in the URL. They've included a double quote that shouldn't be there. Remove it and you can download.
Corrected URL: https://content.arduino.cc/assets/UNOWiFiREV2-reference.zip
Looks like there is a typo in the URL. They've included a double quote that shouldn't be there. Remove it and you can download.
Corrected URL: https://content.arduino.cc/assets/UNOWiFiREV2-reference.zip