Improve readability of lowest unmerged pull request condition

Restructure the "Lowest unmerged pull request" bullet to match the
sentence pattern of the "Top pull request" bullet in the same list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ittakato

content/pull-requests/how-tos/merge-and-close-pull-requests/optimizing-ci-for-stacked-pull-requests.md

@@ -55,7 +55,7 @@ jobs:
 
 Because a workflow runs for every pull request in a stack, you can use the `stack` fields to run expensive jobs only at the positions that matter. Two conditions are especially useful:
 
-* **Lowest unmerged pull request** — the pull request currently at the bottom of the remaining stack. Because it targets the stack base directly, `github.event.pull_request.stack.base.ref` equals `github.event.pull_request.base.ref`.
+* **Lowest unmerged pull request** — the pull request currently at the bottom of the remaining stack, targeting the stack base directly. It is the pull request where `github.event.pull_request.stack.base.ref` equals `github.event.pull_request.base.ref`.
 * **Top pull request** — the last pull request in the stack, containing the full set of changes. It is the pull request where `github.event.pull_request.stack.position` equals `github.event.pull_request.stack.size`.
 
 ```yaml