Update .github/workflows/manual-backport.yml

Author: terrancedejesus

.github/workflows/manual-backport.yml

@@ -81,6 +81,6 @@ jobs:
       - name: "Notify slack on failure"
         uses: craftech-io/slack-action@v1
         with:
-          EXTERNAL_SLACK_DETECTION_RULES_URL: ${{ secrets.EXTERNAL_SLACK_DETECTION_RULES_URL }}
+          slack_webhook_url: ${{ secrets.EXTERNAL_SLACK_DETECTION_RULES_URL }}
           status: failure
         if: failure()