Update _index.md (#11265)

Replacing the invalid (404) promtail-ec2.yaml with the valid one.

**What this PR does / why we need it**:
This PR does - updates the invalid url of the promtail-ec2.yaml with the
valid one.
Why it is needed - [While implementing this
doc](https://grafana.com/docs/loki/latest/send-data/promtail/cloud/ec2/),
the curl won't work.

**Which issue(s) this PR fixes**:
Fixes #<issue number>
No issue has been raised for this
**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] 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](d10549e3ec)
- [ ] 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](0d4416a4b0)

Co-authored-by: J Stickler <julie.stickler@grafana.com>

docs/sources/send-data/promtail/cloud/ec2/_index.md

@@ -95,7 +95,7 @@ Now we're going to download the [Promtail configuration]({{< relref "../../../..
 The file is also available as a gist at [cyriltovena/promtail-ec2.yaml][config gist].
 
 ```bash
-curl https://raw.githubusercontent.com/grafana/loki/main/docs/sources/clients/aws/ec2/promtail-ec2.yaml > ec2-promtail.yaml
+curl https://raw.githubusercontent.com/grafana/loki/main/docs/sources/send-data/promtail/cloud/ec2/promtail-ec2.yaml > ec2-promtail.yaml
 vi ec2-promtail.yaml
 ```