update vague description of only_upgrade (#21050)

* the `only_update` param description was kinda vague:
  * either install if not installed
  * or upgrade if installed
  * or both!?
    * as a result, remove the "install" statement from this sentence

Resolves:
Related:
Signed-off-by: Daniel Andrei Minca <mandrei17@gmail.com>

lib/ansible/modules/packaging/os/apt.py

@@ -116,7 +116,7 @@ options:
     version_added: "2.1"
   only_upgrade:
     description:
-      - Only install/upgrade a package if it is already installed.
+      - Only upgrade a package if it is already installed.
     required: false
     default: false
     version_added: "2.1"