Add TXM deb update instructions

The Ubuntu logitheque does not uninstall TXM when opening the a deb setup file

In the https://txm.gitpages.huma-num.fr/txm-manual/installation.html#installation-avec-la-logith%C3%A8que-ubuntu section : add an optional step to uninstall TXM from the logitheque

or point to the uninstall section (not created yet ?)