docs: Update simple scalable mode documentation by adding quotes around the schema config timestamps

[richardvdveen]

What this PR does / why we need it:
The from field in schema configurations need to be quoted in the Helm values file, otherwise a time portion is added on a Helm install leading to errors. The documentation shows examples without quotes.

Which issue(s) this PR fixes:
No recorded issue, this is a minor fix to the documentation that has been added for other deployment modes already.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• 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
• 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

[CLAassistant]

All committers have signed the CLA.

[JStickler]

Hi @richardvdveen, could you also update line 221 please? And thanks for taking the time to update the docs.