Modify demo installation samples

[DanSGiesbrecht]

When installing either kicad-nightly-demo or kicad-demo, I get the following messages:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package kicad-demo is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However, the following packages replace it:
  kicad-demos

---

~$ sudo apt install kicad-nightly-demo
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package kicad-nightly-demo is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However, the following packages replace it:
  kicad-nightly-demos

[nickoe]

Tank you.