[release-2.9.x] Update _index.md (#11055)

Backport 54320f2 from #11052

---

Changed the title from "Prometheus pipeline stages" to "Promtail
pipeline stages"

**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. <!--
TODO(salvacorts): Add example PR -->

Co-authored-by: steve-caron-grafana <[email protected]>

docs/sources/send-data/promtail/stages/_index.md

@@ -1,13 +1,13 @@
 ---
-title: Prometheus pipeline stages
+title: Promtail pipeline stages
 menuTitle:  Pipeline stages
 description: Overview of the Promtail pipeline stages.
 aliases: 
 - ../../clients/promtail/stages/
 weight:  700
 ---
 
-# Prometheus pipeline stages
+# Promtail pipeline stages
 
 This section is a collection of all stages Promtail supports in a
 [Pipeline]({{< relref "../pipelines" >}}).