Porting document tag and description not matching

# Docs issue template

## Page link
https://os.mbed.com/docs/mbed-os/v6.15/porting/porting-custom-boards.html
## Expected to find...
The device_has_add section changes to add USBDEVICE because the new board use that feature.
## Found
The device_has_add section changes to remove USBDEVICE because the new board doesn't use that feature.