docs: fix run-policy link path

Fixes this warning from typdoc,
```
./packages/SwingSet/src/controller/controller.js:480:21 - [warning] The relative path ../docs/run-policy.md is not a file and will not be copied to the output directory

480         * [`runPolicy`](../docs/run-policy.md) to rate-limit cleanups.
```

packages/SwingSet/src/controller/controller.js

@@ -477,7 +477,7 @@ export async function makeSwingsetController(
      *
      * The first `controller.run()` after this call will delete all
      * the old vat's state at once, unless you use a
-     * [`runPolicy`](../docs/run-policy.md) to rate-limit cleanups.
+     * [`runPolicy`](../../docs/run-policy.md) to rate-limit cleanups.
      *
      * @param {VatID} vatID
      * @param {SwingSetCapData} reasonCD