PR template typo: labeled / labelled (microsoft#21854)

Author: Josh Goldberg

pull_request_template.md

@@ -2,7 +2,7 @@
 Thank you for submitting a pull request!
 
 Here's a checklist you might find useful.
-[ ] There is an associated issue that is labelled
+[ ] There is an associated issue that is labeled
   'Bug' or 'help wanted' or is in the Community milestone
 [ ] Code is up-to-date with the `master` branch
 [ ] You've successfully run `jake runtests` locally