Typo in umpc-ubuntu.sh

[fes111-7]

@flexiondotorg -- First of all, thanks for everything that you and others have done over the years to bring Ubuntu to UMPC devices. Because of you, I am now running Ubuntu 20.04 very successfully on the GPD Pocket 2; a journey that started with Ubuntu MATE 18.10.

There is, however, a typo in the script umpc-ubuntu.sh, which I have used to inject config files into existing installations: the variable TOUCH_RULES is defined at the start of the script, but ${TOUCH_CONF} is used in error in the function disable_umpc_config().

The consequence is that disabling the configuration leaves the touchscreen unusable, as the injected touchscreen config file is not removed by the script.

I would fix this myself, but I'm a noob when it comes to GitHub and I don't want to cock it up!