ci: Fix stalebot config (tendermint#131)

I noticed that stalebot wasn't respecting our `wip` label, only to realize I hadn't yet updated its config.

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#### PR checklist

- [x] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

.github/workflows/stale.yml

@@ -17,4 +17,4 @@ jobs:
           days-before-close: -1
           days-before-pr-stale: 10
           days-before-pr-close: 4
-          exempt-pr-labels: "S:wip"
+          exempt-pr-labels: "wip"