[7.16] Fix inventory threshold rule preview description (backport #3941)

[mergify]

Closes #3880

[mergify]

Cherry-pick of 17d170b has failed:

On branch mergify/bp/7.16/pr-3941
Your branch is up to date with 'origin/7.16'.

You are currently cherry-picking commit 17d170ba.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/en/observability/images/inventory-alert.png
	deleted by us:   docs/en/observability/inventory-threshold-alert.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[dedemorton]

Closing because backporting to this branch is not required as indicated in our backporting policy.