[release-2.9.x] docs: fix runtime_config example (#11826)

Backport 9d4d101 from #11798

---

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

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

**Special notes for your reviewer**:

**Checklist**
- [ ] 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/setup/upgrade/_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)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](0d4416a)

Co-authored-by: Meng Ye <[email protected]>

docs/sources/configure/_index.md

@@ -4818,7 +4818,8 @@ place in the `limits_config` section:
 configure a runtime configuration file:
 
     ```
-    runtime_config: overrides.yaml
+    runtime_config:
+      file: overrides.yaml
     ```
 
     In the `overrides.yaml` file, add `unordered_writes` for each tenant

docs/sources/configure/index.template

@@ -152,7 +152,8 @@ place in the `limits_config` section:
 configure a runtime configuration file:
 
     ```
-    runtime_config: overrides.yaml
+    runtime_config:
+      file: overrides.yaml
     ```
 
     In the `overrides.yaml` file, add `unordered_writes` for each tenant