Docs: Fix promtail installation doc (#8007)

**What this PR does / why we need it**:
Fix promtail installation command using helm in the docs.

**Which issue(s) this PR fixes**:
None

**Special notes for your reviewer**:
None

**Checklist**
- [X] Reviewed the `CONTRIBUTING.md` guide
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

Signed-off-by: Ammar Lakis <ammar.lakis@gmail.com>

docs/sources/clients/promtail/installation.md

@@ -37,7 +37,7 @@ helm repo update
 Finally, Promtail can be deployed with:
 
 ```bash
-$ helm upgrade --install promtail grafana/promtail --set "loki.serviceName=loki"
+helm upgrade --install promtail grafana/promtail
 ```
 
 ## Kubernetes