docs(debian): clarify override behavior for `apt_pref` and `apt_upgr`

plugins/debian/README.md

@@ -13,7 +13,12 @@ plugins=(... debian)
 - `$apt_pref`: use aptitude or apt if installed, fallback is apt-get.
 - `$apt_upgr`: use upgrade or safe-upgrade (for aptitude).
 
-Set `$apt_pref` and `$apt_upgr` to whatever command you want (before sourcing Oh My Zsh) to override this behavior.
+Set **both** `$apt_pref` and `$apt_upgr` to whatever command you want (before sourcing Oh My Zsh) to override this behavior, e.g.:
+
+```sh
+apt_pref='apt'
+apt_upgr='full-upgrade'
+```
 
 ## Common Aliases