add "issue addressed" prompt (#17646)

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,6 +15,7 @@ Thanks again!
 <!-- 
 - If there's an existing issue for your change, please link to it.
 - If there's _not_ an existing issue, please open one first to make it more likely that this update will be accepted: https://github.com/github/docs/issues/new/choose. -->
+**Closes [issue link]**
 
 ### What's being changed: