use URL escape codes

Signed-off-by: Matthew Fisher <matt.fisher@microsoft.com>

.github/workflows/stale-issue-bot.yaml

@@ -10,6 +10,6 @@ jobs:
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         stale-issue-message: 'This issue has been marked as stale because it has been open for 90 days with no activity. This thread will be automatically closed in 30 days if no further activity occurs.'
-        exempt-issue-labels: 'keep open,v4.x'
+        exempt-issue-labels: 'keep+open,v4.x'
         days-before-stale: 90
         days-before-close: 30