Fix needs_release_notes.yml workflow (#3824)

Remove a leading "-" character that violated github-actions-flavored yaml syntax.
This should prevent email spam from failing workflows, and also prevent the workflow itself
from failing.

Author: d-v-b

.github/workflows/needs_release_notes.yml

@@ -1,8 +1,8 @@
 name: "Pull Request Labeler"
 
 on:
-  - pull_request_target:
-      types: [opened, reopened, synchronize]
+  pull_request_target:
+    types: [opened, reopened, synchronize]
 
 jobs:
   labeler: