Fix migration guide redirect link (#6229)

### What

This link was never actually used anywhere except when navigating to it,
in which case it was broken.

See https://rerun.io/docs/reference/migration-0-9 which briefly shows a
page, then redirect you to https://rerun.io/docs/migration/migration-0-9
which is a 404. It should be `reference/migration/migration-0-9`.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6229?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6229?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/6229)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

docs/content/reference/migration-0-9.md

@@ -1,6 +1,6 @@
 ---
 title: Migrating from 0.8 to 0.9
 order: 13
-redirect: migration/migration-0-9
+redirect: reference/migration/migration-0-9
 hidden: true
 ---