update secrets syntax for pr-merge action

.github/workflows/pr-merge.yaml

@@ -8,6 +8,8 @@
         runs-on: ubuntu-latest
         steps:
         - name: Send Discord notification
+          env:
+            DISCORD_WEBHOOK_BETA_TESTERS: ${{ secrets.DISCORD_WEBHOOK_BETA_TESTERS }}
           if: github.event.pull_request.merged == true
           run: |
-            curl -H "Content-Type: application/json" -d '{"content": "Pull Request #{{ github.event.pull_request.number }} merged by {{ github.actor }}"}' ${{ secrets.DISCORD_WEBHOOK_BETA_TESTERS }}
+            curl -H "Content-Type: application/json" -d '{"content": "Pull Request #{{ github.event.pull_request.number }} merged by {{ github.actor }}"}' $env:DISCORD_WEBHOOK_BETA_TESTERS