docs: Change prefix for desktop applications to app-

docs: Change prefix for desktop applications to app-

We need both a slice name and a prefix for application units. For
consistency we tried to use the same name and ended up standardising on
"apps.slice" and and "apps-" prefix for the units.

However, "app-" would be a more natural prefix for applications. And it
is no problem to simply also name the slice "app.slice" for consistency
rather than keeping the current "apps.slice".