correct the pr slot name pattern in cleanup job

.github/workflows/pr-cleanup.yml

@@ -4,12 +4,12 @@ on:
   pull_request:
     types: [closed]
 
-concurrency: 
+concurrency:
   group: ci-${{ github.event.number }}
   cancel-in-progress: true
 
 env:
-  SLOT_NAME: preview-${{ github.event.number }}
+  SLOT_NAME: pr-${{ github.event.number }}
 
 jobs:
   delete-slot:
@@ -20,7 +20,7 @@ jobs:
       uses: azure/login@v1
       with:
         creds: ${{ secrets.AZURE_CREDENTIALS }}
-    
+
     - name: Delete slot on staging site
       run: az webapp deployment slot delete --resource-group ${{ vars.AZURE_RESOURCE_GROUP }} --name ${{ vars.AZURE_WEBAPP_NAME}} --slot ${{ env.SLOT_NAME }}
 
@@ -33,4 +33,4 @@ jobs:
         with:
           environment: "PR #${{ github.event.number }}"
           token: ${{ secrets.GITHUB_TOKEN }}
-          onlyRemoveDeployments: true
+          onlyRemoveDeployments: true