Jsonet: Fixes typo in shipper.libjsonnet (#9992)

**What this PR does / why we need it**:

Changes line 43 in production/ksonnet/loki shipper.json from:

`shared_store: if self.using_tsdb_shipper then
self.tsdb_shipper_shared_store else self.boltdb_shipper_shared_store`,

to

`shared_store: if $._config.using_tsdb_shipper then
$._config.tsdb_shipper_shared_store else
$._config.boltdb_shipper_shared_store,`

In order to fix "field using_tsdb_shipper not found" 

**Which issue(s) this PR fixes**:
Fixes #<issue number>

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e)

Co-authored-by: Michel Hollands <42814411+MichelHollands@users.noreply.github.com>

production/ksonnet/loki/shipper.libsonnet

@@ -40,7 +40,7 @@
       compactor+: {
         working_directory: '/data/compactor',
         // prefer tsdb index over boltdb
-        shared_store: if self.using_tsdb_shipper then self.tsdb_shipper_shared_store else self.boltdb_shipper_shared_store,
+        shared_store: if $._config.using_tsdb_shipper then $._config.tsdb_shipper_shared_store else $._config.boltdb_shipper_shared_store,
       },
     } else {},
   },