Use sha instead of ref

missed one.

Author: MarkDaoust

.github/workflows/bot-pr-new.yaml

@@ -22,7 +22,7 @@ jobs:
       uses: actions/checkout@v2
       with:
         repository: ${{ github.event.client_payload.pull_request.head.repo.full_name }}
-        ref: ${{ github.event.client_payload.pull_request.head.ref }}
+        ref: ${{ github.event.client_payload.pull_request.head.sha }}
     - name: Fetch base master branch
       run: git fetch -u "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY" master:master
     - name: Create message