Fix testplan-item label spelling

.github/workflows/test-plan-item-validator.yml

@@ -16,5 +16,5 @@ jobs:
         uses: ./test-plan-item-validator
         with:
           token: ${{secrets.VSCODE_ISSUE_TRIAGE_BOT_PAT}}
-          label: testplan-items
+          label: testplan-item
           comment: Invalid test plan item. See errors below and the [test plan item spec](https://github.com/microsoft/vscode/wiki/Writing-Test-Plan-Items) for more information. This comment will go away when the issues are resolved.